Inclusion Criteria:  Willing to sign the ICF and follow the requirements specified in the protocol. Age: ≥18 years, both genders. Expected survival time≥3 months. Patients with histologically confirmed unresectable recurrent or metastatic squamous cell carcinoma of head and neck. Failed in the prior platinum and/or anti-PD-1 treatment. Patients must have measurable lesions according to the Response Evaluation Criteria in Solid Tumors (RECIST v1.1). ECOG performance score 0 or 1. Organ functions and coagulation function must meet the basic requirements. No severe cardiac dysfunction with left ventricular ejection fraction (LVEF) ≥ 50%. Serum or urine pregnancy test negative within 7 days before the first dose of investigational drug. Patients with childbearing potential must use effective contraception during the treatment and for 6 months after the last dose of treatment.  Exclusion Criteria:  History of 4 or more systemic anti-tumor therapies for the recurrent or metastatic squamous cell carcinoma of head and neck. Expected surgery or any other form of systemic or local anti-tumor therapy. History of systemic chemotherapy within 3 weeks before the first administration of the investigational drug, targeted small molecule therapy within 2 weeks or 5 half-life periods before the first administration (whichever is shorter), antitumor biological therapy or immunotherapy within 4 weeks before the first administration, or major surgery. Known active CNS metastasis and/or cancerous meningitis. Residual toxicity reactions caused by previous anti-tumor treatment or abnormal values of laboratory tests higher than grade 1 (CTCAE v5.0). Uncontrolled or poorly controlled heart disease. History of pulmonary embolism or deep vein thrombosis within 3 months before the first administration of the investigational drug. Known history of malignancy. Uncontrolled or poorly controlled hypertension. Patients with a history of active bleeding, coagulopathy, or receiving coumarin anticoagulation therapy. Known allergic reaction to any ingredients or excipients of MRG003. Known active hepatitis B or C. Complicated with severe, uncontrolled infection or known human immunodeficiency virus (HIV) infection, or diagnosed as acquired immunodeficiency syndrome (AIDS); or uncontrolled autoimmune disease; or history of allogeneic tissue/organ transplantation, stem cell or bone marrow transplantation, or solid organ transplantation. Active bacterial, viral, fungal, rickettsia, or parasitic infections that require systemic anti-infective treatment. Vaccination of live virus vaccine within 30 days before the first administration of the study drug. Inactivated seasonal influenza vaccine or approved COVID-19 vaccine is allowed. Moderate to severe dyspnea at rest caused by advanced cancer or its complications, or severe primary lung disease, oxygen saturation < 93% in non-oxygen state, or history of any interstitial lung disease or interstitial lung disease (ILD) requiring oral or intravenous glucocorticoids or non-infectious pneumonia. Patients receiving immunology-based treatment for any reason. Uncontrolled pleural effusion, pericardial effusion or recurrent ascites. Potent CYP3A4 inhibitors or inducers are in use and cannot be discontinued. Women who are lactating or pregnant. Other conditions that in the clinical judgement of the investigator make the patient not suitable for participation in this study.