This is a phase II, multi-center, open label, single arm, interventional study. Patients with HER2-positive metastatic gastric or gastro-esophageal junction adenocarcinoma who have not received prior treatment for metastatic disease will be treated with trastuzumab(8 mg/kg loading dose followed by 6 mg/kg every 3 weeks ),Capecitabine(2000mg/m2d, d1-14,every 3 weeks) and Docetaxel (60mg/m2 every 3 weeks for 6 cycles).All patients will continue to receive trastuzumab and Capecitabine until either disease progression, occurrence of unacceptable toxicity or withdrawal from the study for another reason.Primary endpoints is PFS and secondary endpoints are ORR, OS and Safety.Recruitment period:24 months;PFS follow-up period: 80% PFS events;OS follow-up period: 18 months or 80% OS events, whichever occurs first.