This is a randomized, double-blind, multicenter trial，parallel control designed to evaluate treatment with trastuzumab + QL1209 + docetaxel compared with trastuzumab + pertuzumab + docetaxel in the participants with early-stage or locally advanced HER2-positive and estrogen receptor/progesterone receptor negative breast cancer. The anticipated treatment duration is approximately 140 days.

This is A multi-center, randomized, double-blind, parallel control，comparative clinical trial.

The primary objective is to evaluate whether the clinical efficacy of QL1209 and pertuzumab are similar in patients with early-stage or locally advanced HER2-positive and estrogen receptor/progesterone receptor negative breast cancer.

The secondary objective are to evaluate whether the clinical safety and immunogenicity of QL1209 and pertuzumab are similar in patients with early-stage or locally advanced HER2-positive and estrogen receptor/progesterone receptor negative breast cancer.

Inclusion Criteria:

1. Volunteer to participate in this clinical study; Completely understand and know this study as well as sign the informed consent form (ICF);
2. Age ≥ 18 years and ≤ 80 years when ICF is signed;
3. Histologically confirmed invasive breast carcinoma with a primary tumor size of more than (>) 2 centimeters (cm) by standard local assessment technique; Breast cancer stage at presentation: early-stage (T2-3, N0-1, M0) or locally advanced (T2-3, N2 or N3, M0; T4, any N, M0); HER2-positive breast cancer confirmed by immunohistochemistry or HER2 gene amplification by in situ hybridization; Estrogen receptor and Progesterone receptor negative.
4. Eastern Cooperative Oncology Group Performance Status equal to or less than (<=) 1.
5. Baseline left ventricular ejection fracture >= 55% measured by echocardiography (preferred) or multiple gated acquisition scan

Exclusion Criteria:

1. Stage IV metastatic breast cancer;
2. Inflammatory breast cancer;
3. Previous anti-cancer therapy or radiotherapy for any malignancy;
4. History of malignancies other than colorectal cancer within 5 years prior to randomization, excluding cutaneous basal cell carcinoma, cervical carcinoma in situ, and thyroid papillary adenocarcinoma of non-melanoma after radical treatment;
5. Concurrent anti-cancer treatment in another investigational trial, including hormone therapy, bisphosphonate therapy, or immunotherapy;
6. Major surgical procedure within 4 weeks prior to randomization or from which the participant has not fully recovered;
7. Serious cardiac illness or medical condition;
8. Other concurrent serious diseases that may interfere with planned treatment, including severe pulmonary conditions/illness;
9. Sensitivity to any of the study medications, any of the ingredients or excipients of these medications, or benzyl alcohol;
10. Pregnant or lactating