This is a randomized, double-blinded, multicenter phase 3 clinical trial to evaluate the efficacy and safety of pyrotinib plus trastuzumab plus docetaxel versus placebo plus trastuzumab plus docetaxel in patients who have HER2 positive metastatic breast cancer and have not received systemic anticancer therapy for advanced disease.

Inclusion Criteria:

1. HER2 positive recurrent or metastasis breast cancer.
2. Patients with measurable disease are eligible.
3. Eastern Cooperative Oncology Group(ECOG) performance status 0 or 1.
4. Adequate organ function.
5. Signed, written inform consent obtained prior to any study procedure.

Exclusion Criteria:

1. History of anti-cancer therapy for MBC（with the exception of one prior hormonal regimen for MBC）.
2. History of HER tyrosine kinase inhibitors or monoclonal antibody for breast cancer in any treatment setting，except trastuzumab used in the neo-adjuvant or adjuvant setting.
3. Assessed by the investigator to be unable receive systemic chemotherapy.
4. History of other malignancy within the last 5 years，except for carcinoma in situ of cervix，basal cell carcinoma or squamous cell carcinoma of the skin that has been previously treated with curative intent.
5. Pregnant or lactating women，or for women of childbearing potential unwilling to use a highly-effective contraception during of the study treatment and for at least 7 months after the last dose of study treatment.