This is a single ascending dose study of 9MW3811, the primary objective of which is to evaluate the safety, tolerability and preliminary efficacy of 9MW3811 in patients with advanced solid tumors.

Inclusion Criteria:

1. Male or female participants between 18 and 75 years of age, inclusive.
2. Histologically or cytologically confirmed advanced malignant solid tumors, for which standard therapy does not exist or has proven ineffective or intolerable.
3. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.
4. Life expectancy of ≥ 3 months.
5. Participants must have measurable disease according to RECIST (version 1.1).
6. Adequate organ functions.
7. Sexually active fertile participants, and their partners, must agree to use methods of contraception during the study and at least 6 months after termination of study therapy.

Exclusion Criteria:

1. Participants with cancerous meningitis and/or central nervous system metastases with clinical symptoms.
2. History of other active malignant tumor within 3 years prior to screening.
3. Suffering from poorly controlled body cavity effusion.
4. Suffering from active autoimmune disease.
5. History of chronic obstructive pulmonary disease (COPD), pulmonary fibrosis, or other respiratory diseases that require hospitalization within 4 weeks prior to the first dose of study drug.
6. History of clinically significant cardiac or cerebrovascular diseases within 6 months prior to the first dose of study drug.
7. History of other severe or uncontrolled systemic disease, i.e. poorly controlled diabetes.
8. Previously received allogeneic hematopoietic stem cell transplantation or solid organ transplantation.
9. Major surgery within 28 days prior to the first dose of study drug.
10. Participants with one or more clinically significant positive test results of hepatitis B surface antigen (HBsAg), hepatitis C virus (HCV) antibody, treponema pallidum antibody or human immunodeficiency virus (HIV) antibody.
11. Participants who have received treatment with biotherapy, endocrine therapy, immunotherapy, or other anti-tumor therapy within 2 weeks prior to the first dose of study drug; Radical radiotherapy received within 3 weeks or palliative radiotherapy received within 2 weeks prior to the first dose of study drug; Received treatment with chemotherapy within 3 weeks prior to the first dose of study drug (6 weeks for nitrosourea or mitomycin); Received treatment with oral fluorouracil or small molecule targeted drugs within 2 weeks or 5 half-lives prior to the first dose of study drug (whichever is shorter); Received treatment with anti-tumor traditional Chinese medicine within 1 week prior to the first dose of study drug; Participated in other clinical trials within 4 weeks prior to the first dose of study drug.
12. Participants who have received systemic treatment with immunosuppressants within 2 weeks prior to the first dose of study drug.