Objective: To investigate the efficacy and safety of mecapegfilgrastim for chemotherapy-induced neutropenia in patients with breast cancer who received postoperative adjuvant chemotherapy. Methods: This was a retrospective study. A total of 178 eligible patients with breast cancer received postoperative epirubicin combined with cyclophosphamide adjuvant chemotherapy in the Breast and Thyroid Surgery Department of Zibo Central Hospital from April 1, 2020 to February 20, 2021. The patients received subcutaneous injection of 6 mg of mecapegfilgrastim 48 h after the first chemotherapy and subsequent chemotherapy. Blood routine was dynamically monitored. The incidence of neutropenia after each chemotherapy, the incidence of grade IV neutropenia, the time to the lowest value of neutrophils after each chemotherapy, the time for neutrophils to apply pegylated recombinant human granulocyte colony-stimulating factor (G-CSF) to recover to >2×109/L, and the occurrence of adverse reactions (such as bone pain, myalgia and fever) were counted. Results: Of the 178 patients with breast cancer, 3 patients completed only 3 cycles of chemotherapy, and the rest completed 4 cycles of chemotherapy, and all patients received prophylactic use of mecapegfilgrastimin after chemotherapy. After all chemotherapy cycles, the time for neutropenia to reach the nadir was (10.15±1.21) d. The incidence was 6.77% of neutropenia and 2.12% of grade IV neutropenia. The time for neutrophils to apply G-CSF to recover to >2×10⁹/L was (1.85±1.34) d. From the first to the fourth chemotherapy cycle, 45, 43, 47, 46 patients developed bone pain and myalgia; 15, 10, 11, and 7 patients developed fever, respectively, who were relieved after symptomatic treatment. Conclusion: Prophylactic use of mecapegfilgrastim after chemotherapy in patients with breast cancer can significantly reduce the incidence of neutropenia and FN, improve the safety of chemotherapy, and is worthy of clinical promotion.