This study consists of two parts. Phase Ia is a dose escalation study to determine the maximum tolerated dose (MTD) and recommended phase II dose (RP2D) of MRG001. Phase Ib is a dose expansion study to assess the preliminary efficacy of MRG001 in patients with CD20-positive relapsed or refractory B-cell NHL at the confirmed RP2D. The safety, tolerability, pharmacokinetic (PK) and immunogenicity of MRG001 will be evaluated in both parts.

Inclusion Criteria:

1. Voluntarily participates in the clinical study; Fully understands and informed of this study, and provides written informed consent; Willing to follow and have the ability to complete all trial procedures.
2. Aged 18 to 80 (including 18 and 80), male or female.
3. Patients with histopathologically confirmed relapsed/refractory B-cell non-Hodgkin lymphoma.
4. Relapsed or refractory disease after standard of care treatment with anti-CD20 antibodies.
5. Patients must have at least one measurable lesion without prior local therapy according to Lugano 2014 criteria.
6. The score of ECOG for performance status is 0 or 1.
7. Patients without severe hematopoietic, liver and kidney dysfunction.
8. Expected survival time ≥ 3 months.
9. Prior anti-tumor treatment-related AEs (NCI CTCAE v5.0 Criteria) have recovered to ≤ Grade 1.
10. Negative blood pregnancy test for women of childbearing potential within 7 days prior to the first dose of study drug. Patients of childbearing potential should agree to use effective contraception from signing the ICF until 3 months after the last dose of study drug.
11. The score of left ventricular ejection fraction (LVEF) is > 50%.

Exclusion Criteria:

1. Applicable to Phase Ia: positive hepatitis B surface antigen (HBsAg), or negative HBsAg with a peripheral blood hepatitis B virus DNA copy number greater than the upper limit of normal; positive hepatitis C virus (HCV) antibody and positive HCV RNA at screening. Patients with a history of clinically significant non-viral hepatitis and cirrhosis. Applicable to Phase Ib: positive hepatitis B surface antigen (HBsAg) and/or positive hepatitis B core antibody (HBcAb) at screening, and peripheral blood hepatitis B virus DNA copy number greater than the upper limit of normal; positive hepatitis C virus (HCV) antibody and positive HCV RNA. Patients with a history of clinically significant non-viral hepatitis and cirrhosis (decompensated cirrhosis Child-Pugh class B, C).
2. Positive human immunodeficiency virus (HIV) antibody.
3. Any active infection requiring systemic therapy occurred within 2 weeks before enrollment.
4. Suspected or confirmed central nervous system invasion of NHL.
5. Allergic constitution or known hypersensitivity to rituximab or other anti-CD20 monoclonal antibodies and their components, or hypersensitivity to any component of MRG001.
6. Patients with uncontrolled or significant cardiovascular disease.
7. History of severe pulmonary disease.
8. Received CAR-T therapy within 3 months before enrollment.
9. Received blood transfusion within 28days before enrollment, or receiced erythropoietin (EPO), granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF) and other drugs that may affect hemogram within 14 days before enrollment.
10. Current use of potent CYP3A4 inhibitors or inducers.
11. History of allogeneic stem cell transplantation or organ transplantation, or have received autologous stem cell transplantation within 3 months prior to screening, or plan to undergo stem cell transplantation.
12. Suffered from other malignancies within the last 5 years.
13. Major surgery within 6 weeks prior to enrollment or planned major surgery within the first 12 weeks after receiving study drug.
14. Patients who are receiving any approved or investigational anti-tumor treatment with antibody/fusion protein/ADC drugs, radiotherapy, chemotherapy, Chinese patent medicine or Chinese herbal medicine within 28 days before enrollment; or have participated in any approved or investigational small molecule targeted therapy within 28 days before enrollment or within 5 half-lives (whichever is shorter).
15. Received any vaccine within 28 days prior to enrollment or will receive any vaccine within 6 months after the last dose of study drug.
16. Requires treatment with glucocorticoids or other immunosuppressive agents for a certain condition within 14 days prior to enrollment.
17. An acute oncolytic reaction may occur at the discretion of the investigator.
18. Inability to complete protocol-specified study visits and study drug administration.
19. Women during pregnancy or lactation; men and women of childbearing potential who are unwilling to take prescribed appropriate contraceptive measures.
20. Other conditions inappropriate for participation in this clinical trial at the discretion of the investigator.
21. Malignant lymphoma with pathological transformation.
22. High-grade B-cell lymphoma.
23. Patient's Body Mass Index ≤ 17 kg/m2.
24. Patients with underlying medical conditions that, in the judgment of the investigator, might increase the risk of receiving study drug or might affect the safety assessment of the study drug.