Pyrotinib is an oral tyrosine kinase inhibitor targeting both HER-1 and HER-2 receptors. This study is a randomized, open-label, multi-center, parallel design study of the combination of pyrotinib, trastuzumab and chemotherapy versus trastuzumab and chemotherapy in HER2+ MBC patients, who have prior received trastuzumab and pyrotinib. Patients will be randomized in a 2:1 ratio to one of the following treatment arms. Arm A: pyrotinib + trastuzumab + chemotherapy, Arm B: trastuzumab + chemotherapy. Patients will receive either arm of therapy until disease progression, unacceptable toxicity, or withdrawal of consent.

Inclusion Criteria:

1. Aged ≥18 and ≤75 years;
2. Pathologically confirmed HER2 positive patients with recurrence/ metastatic breast cancer: HER2 IHC 3+, or HER2 IHC 2+ and FISH detection gene amplification;
3. History of trastuzumab-containing chemotherapy in neoadjuvant, adjuvant or recurrence/ metastatic setting;
4. History of pyrotinib-containing chemotherapy in neoadjuvant or recurrence/ metastatic setting;
5. Previously reveived ≤2 systemic treatment in recurrence/ metastasis setting; (anti-HER2 ADCs such as T-DM1 is included in chemotherapy regimens, endocrine therapy alone is not included);
6. ECOG performance status of 0 to 1;
7. According to RECIST 1.1, at least one extracranial measurable lesion exists；
8. Signed informed consent.

Exclusion Criteria:

1. Patients with leptomeningeal metastasis or unstable brain metastasis;
2. History of neurological or psychiatric disorders;
3. Second malignancies within 5 years, except for cured skin basal cell carcinoma, carcinoma in-situ of uterine cervix and squamous-cell carcinoma;
4. Undergone major surgical procedures or significant trauma within 4 weeks prior to randomization, or expected to undergo major surgery.
5. Factors influencing the usage of oral administration (e.g. unable to swallow, chronic diarrhea and intestinal obstruction, etc.);
6. History of allergies to the drug components of this regimen;
7. History of Immunodeficiency, acquired or congenital immunodeficiency (HIV positive), history of organ transplantation;
8. Any other situations judged by investigator as not suitable for participating in this study.