This is a single arm, open-label, dose escalation, PK expansion and efficacy expansion study of phase I. The purpose of this study is to assess the tolerability, safety, pharmacokinetics and immunogenicity of SHR-A1811 and preliminary anti-tumor efficacy in HER2-expressing advanced gastric or gastroesophageal junction adenocarcinoma and colorectal cancer.

Inclusion Criteria:

• Has a pathologically or cytologically documented advanced/unresectable or metastatic gastric or gastroesophageal junction adenocarcinoma and colorectal cancer that is refractory to or intolerable with standard treatment, or for which no standard treatment is available.
• At least one evaluable or measurable lesion should be present and identified according to Response Evaluation Criteria in Solid Tumors(RECIST) version 1.1.
• Has an ECOG PS 0-1.
• Has a life expectancy of ≥ 3 months.
• Has adequate organ function

Exclusion Criteria:

• Has unresolved toxicities from previous anticancer therapy, defined as toxicities not yet resolved to NCI-CTCAE version 5.0 grade ≤ 1.
• Has received HER2 antibody drug conjugates,.
• Central nervous system metastasis or meningeal metastasis with clinical symptoms
• Has active infection requiring systemic treatment.