Background: Targeted therapy has significant efficacy in adjuvant and advanced treatment of HER2-positive breast cancer, and it has also promoted its development in neoadjuvant therapy. For HER2-positive breast cancer, chemotherapy with anthracyclines and taxanes combined with trastuzumab targeted therapy is currently the standard regimen for adjuvant or neoadjuvant therapy. On the basis of the preliminary clinical trials, we carried out this study to confirmed efficacy and safety of pyrotinib combined with trastuzumab and nab-paclitaxel as neoadjuvant treatment regimen in patients(pts) with stage II- III HER2-positive breast cancer. Method: In this multicenter, single arm, prospective, open-label phase II study, 46pts with stage II-III HER2-positive breast cancer will be enrolled (in Simon twostages model, 16 pts will be enrolled in the first stage) to receive pyrotinibcombined with trastuzumab and nab-paclitaxel. The inclusion criteria are asflows. (1) female aging from 18 to 75; (2) Eastern Cooperative Oncology Group(ECOG) performance score of 0-1; (3) Life expectancy is no less than 12weeks;(4) Pathologically confirmed HER-2 positive breast cancer;(5) earlystage (T2-3, N0-1, M0) or locally advanced (T2-4, N2, or N3), with tumor sizeno less than 2cm in diameter. All pts will receive pyrotinib (400mg qd) andtrastuzumab: 8 mg/kg for cycle 1 and 6 mg/kg for cycles 2-6, intravenousinfusion, 21 days for 1 cycle, a total of 6 cycles and nab-paclitaxel: 260mg/m2,intravenous infusion, 21 days as a cycle, a total of 6 cycles. The primaryendpoint is pathological complete response rate (pCR: ypT0/is, ypN0), and thesecondary endpoints are disease-free survival time (DFS), objective responserate (ORR), and survival time without distant metastasis (DDFS). Findings: From Nov 2020 to May 2021, a total of 11 pts were enrolled, 4 pts havecompleted surgery and 6 pts could be evaluated for ORR. The pCR rate was50%, ORR was 100% (5 PR, 1CR). Adverse events (AEs) of any garde werediarrhea 11 (100.0%), anorexia and vomiting 8 (72.7%) and rash 4 (36.4%).Garde 3 AEs were diarrhea 3 (27.3%), and there was no grade 4 AEs. Conclusion: Pyrotinib combined with trastuzumab and nab-paclitaxel in neoadjuvanttreatment for pts with stage II-III HER2-positive breast cancer showedacceptable toxicity and promising efficacy. Citation Format: Bin Shao, Zhengyan Yu, Yu Wang, Guangcai Zhao, Fei Zhou, Ranmei Wei. A prospective, open, multi-center clinical study of pyrotinib combined with trastuzumab and nab-paclitaxel in neoadjuvant treatment for stage II-III HER2-positive breast cancer patients [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-13-37.