Inclusion Criteria:  1. Aged 18 to 75 (including 18 and 75)，both genders;  2. Expected survival time ≥ 6 months;  3. The score of ECOG for performance status is 0 or 1;  4. Patients with histologically and/or cytologically confirmed HER2-positive invasive breast cancer, including unresectable locally advanced breast cancer (LABC) or metastatic breast cancer (MBC);  5. Failed in the prior trastuzumab ± pertuzumab and taxane therapy with advanced/metastatic disease, or progression within 12 months after new adjuvant or adjuvant therapy; or those who have failed in the prior TKI and/or HER2-ADC therapy can also be enrolled;  6. Patients must have imaging evidence of tumor progression during or after the most recent treatment confirmed by the investigator and at least one measurable lesion baseline according to the Response Evaluation Criteria In Solid Tumors (RECIST 1.1);  7. Organ functions must meet the basic requirements;  8. Willing to sign the ICF and follow the requirements specified in the protocol; Reproductive male and female subjects of childbearing age shall be willing to take effective contraceptive measures from the date of signing the ICF to 6 months after the last dose; Women of childbearing potential must have a negative pregnancy test within 7 days before the first dose.  Exclusion Criteria:  1. History of other primary malignancies;  2. Received other study drugs for clinical trials, any anti-tumor drugs/ biological or investigational therapeutic drug, radiotherapy, anti-tumor vaccine therapy, etc., within 4 weeks prior to the first dose;  3. Presence of central nervous system (CNS) metastasis;  4. The pleural or peritoneal effusion with combined clinical symptoms, which seriously endangers the life safety of the subjects or urgently needs clinical treatment. Or the pericardial effusion with combined clinical symptoms;  5. Any severe or uncontrolled systemic disease, uncontrolled heart disease, uncontrolled diabetes, and active bleeding signs judged by the investigator;  6. Evidence of active infection, including hepatitis B, hepatitis C or human immunodeficiency virus (HIV) infection, active bacteria infection requiring systematic anti-infective therapy, infection caused by other viruses, fungi, rickettsia or parasites;  7. History of hypersensitivity to any component of MRG002 or history of hypersensitivity of ≥ Grade 3 to trastuzumab injection;  8. Subjects with active autoimmune disease or a history of autoimmune disease are receiving immunosuppressive agents or systemic hormone therapy, and are still receiving within 2 weeks prior to enrollment;  9. History of interstitial pneumonia, severe chronic obstructive pulmonary disease, severe pulmonary insufficiency, symptomatic bronchospasm, etc;  10. Other conditions inappropriate for participation in this study, as deemed by the investigator.