This is a phase 1/2 multicenter, first-in-human study of IBI129. It includes a phase 1 dose escalation and expansion section to identify MTD/RP2D of IBI129, plan to enroll 22~180 subjects, and a phase 2 to explore efficacy, safety and tolerability of IBI129 at RP2D in specified types of solid tumor. Approximately 182 evaluable subjects will be enrolled for phase 2

Inclusion Criteria:

1. Subjects with the ability to understand and give written informed consent for participation in this trial, including all evaluations and procedures as specified by this protocol;
2. At least 1 evaluable lesion according to Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1.;
3. Male or female subjects ≥ 18 years old;
4. Eastern Cooperative Oncology Group (ECOG) performance status (PS) of 0 or 1;
5. Anticipated life expectancy of ≥ 12 weeks;
6. Adequate bone marrow and organ function

Exclusion Criteria:

1. Participate in any other interventional clinical research except observational (non-interventional) study or in the follow-up phase of the interventional study;
2. Received previous anti-tumor therapy within 4 weeks or 5 half-lives of the anti-tumor regimens before the first administration of study drug, whichever is shorter.
3. Progressed refractory to an antibody drug conjugate that consists of an exatecan derivative that is a topoisomerase I inhibitor.
4. Plan to receive other antitumor therapy during the study excluding palliative radiotherapy for the purpose of symptom (like pain) relief that must also do not have impact on tumor assessment throughout the study;
5. Known symptomatic central nervous system (CNS) metastases.