The goal of this clinical trial is to learn the safety, tolerability, pharmacokinetic characteristics and efficacy of FC084CSA in combination with Tislelizumab in patients with advanced malignant solid tumors.

The study includes two phases. Phase Ib adopts a "3+3" dose escalation design to assess safety and tolerability of increasing dose levels of FC084CSA in combination of fixed dose of Tislelizumab. Phase IIa is the dose expansion phase to further observe the preliminary effectiveness of the recommended Phase 2 Dose of FC084CSA in combination of Tislelizumab.

Inclusion Criteria:

1. Aged 18 to 75 years old male and female.
2. Phase Ib: Patients with histologically or cytologically diagnosed solid tumors who have failed standard therapy; Phase IIa: Patients with histologically or cytologically confirmed stage IIIB/IIIC and stage IV NSCLC which surgery or radiotherapy cannot be performed.
3. No known sensitizing mutations or other actionable oncogenes with approved therapies if available.
4. Prior PD-1/PD-L1 inhibitor combined with platinum-containing therapy failed;
5. According to RECIST 1.1, there is at least one measurable lesion.
6. ECOG performance status 0-1.
7. Major organs are functioning well.

Exclusion Criteria:

1. Not recovered from the adverse reactions caused by previous anti-tumor treatments (≥CTCAE grade 1).
2. Received anti-tumor therapy within 4 weeks before enrollment.
3. Participated in other clinical trials within 4 weeks before enrollment and used clinical investigational drugs during this period.
4. Have undergone surgery within 4 weeks before enrollment, and the investigator believes that the patient's state has not recovered to the point where the study can be started.
5. Patients with ascites (ascites), pleural effusion (pleural effusion) or pericardial effusion that cannot be controlled by drainage or other methods.
6. Central nervous system metastases with clinical symptoms.
7. With any situations that the researcher considers inappropriate to participate in this research.