• 1\) Age 18\

∙ 75 years old (including critical mass), gender is not limited. 2) Phase II cohort 1 and III only: patients with locally advanced or metastatic gastric cancer (including adenocarcinoma of the gastro-esophageal junction) diagnosed by histopathology, unsuitable for radical surgical resection or localized treatment, and who have not received systemic antitumor therapy (including systemic chemotherapy, molecularly-targeted drug therapy, biologic therapy, and other investigational therapeutic agents) for GC (except for adjuvant chemotherapy for \>6 months), and who have demonstrated disease progression; and patients who have been diagnosed by immunohistochemistry (IHC) staining and/or fluorescence in situ hybridization (FISH). and demonstrated disease progression excepted); HER2 positivity (IHC 3+, or IHC 2+ and FISH +) demonstrated by immunohistochemical (IHC) staining and/or fluorescence in situ hybridization (FISH).

• 3\) Phase II Cohort 2 only: Have histologically or cytologically confirmed advanced malignant solid tumors that have failed standard treatment, or for which no standard treatment options are available, or for which standard treatment is not applicable at this stage; and are HER2 underexpressed (IHC 2+ and FISH-, or IHC 1+) as evidenced by immunohistochemistry (IHC) staining and/or fluorescence in situ hybridization (FISH).

• 4\) Phase II Cohort 3 only: with locally advanced or metastatic gastric cancer (including gastro-oesophageal junction adenocarcinoma) or colorectal cancer diagnosed by histopathology, unsuitable for radical surgical resection or localized treatment, with no prior systemic (including systemic chemotherapy, molecularly-targeted drug therapy, biologic therapy, and other investigational therapeutic agents) antitumor therapy (having received adjuvant chemotherapy for \>6 months with evidence of disease progression), patients with wild-type KRAS, NRAS, and BRAF genes (mCRC only); and HER2 low expression (IHC 2+ and FISH-, or IHC 1+) demonstrated by immunohistochemical (IHC) staining and/or fluorescence in situ hybridization (FISH).

• 5\) At least 1 measurable lesion according to RECIST 1.1 criteria (tumor lesions located in the area of prior radiotherapy or other localized regional treatment sites are generally not considered measurable lesions unless the lesion shows definite progression or persists after three months of radiotherapy).

• 6\) Eastern Cooperative Oncology Group (ECOG) physical status score of 0 to 1. 7) Have an expected survival of ≥ 3 months. 8) Adequate organ function:

⁃ Hematologic system (no transfusion or hematopoietic stimulating factor therapy within 14 days): absolute neutrophil count (ANC) ≥ 1.5 × 109/L, platelet count (PLT) ≥ 90 × 109/L, hemoglobin (HGB) ≥ 90 g/L; Liver function: total bilirubin (TBIL) ≤ 1.5 times the upper limit of normal (ULN), except Gilbert's syndrome; aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 3.0 times the ULN, liver metastasis or hepatocellular carcinoma patients need to AST and ALT ≤ 5.0 times the ULN and total bilirubin ≤ 3.0 times the ULN; Renal function: serum creatinine (Cr) ≤1.5 times ULN; if creatinine \>1.5 times ULN, creatinine clearance (Ccr) ≥50 mL/min (calculated according to Cockcroft-Gault formula);

‣ ④ Coagulation function: International Normalized Ratio (INR) ≤ 1.5 times ULN for prothrombinogen, Activated Partial Thromboplastin Time (APTT) ≤ 1.5 times ULN, or INR and APTT ≤ 2.5 times ULN for patients with liver metastasis or hepatocellular carcinoma.

‣ 9\) Eligible patients (male and female) of childbearing potential must agree to use a reliable method of contraception (hormonal or barrier method or abstinence) with their partner for the duration of the trial and for at least 6 months after the last dose; female patients of childbearing potential must have a negative serum pregnancy test within 7 days prior to the first dose of study drug.

‣ 10\) Subjects must give informed consent for this study prior to the trial and voluntarily sign a written informed consent form.