Background: Neutropenia is the frequently observed adverse events for breast cancer patients under chemotherapy. Prophylactic administration of mecapegfilgrastim could reduce the incidence of grade 3/4 neutropenia/febril neutropenia(FN). The effectiveness of mecapegfilgrastim in breast cancer patients and different subgroups were explored. Methods: Patients with breast cancer and tolerable of mecapegfilgrastim were prospectively enrolled in a real-world study. All the patients received mecapegfilgrastim prophylacticly at least in chemotherapy cycle one. The incidence of grade 3/4 neutropenia/febril neutropenia in all patients and subgroups were presented. Results: 322 eligible patients were enrolled. The median age was fifty one years. The ECOG performace score was 0 – 1. In total, 752 chemothrapy cycles were conducted. Grade 3/4 neutropenia occurred in 44 (5.9 %) cycles, grade 4 neutropenia occurred in 16 (2.1 %) cycles. FN were reported in 4 (1.2 %) patients. The incidence of grade 3/4 neutropenia were stratificated by age group (≥65, ＜65), chemotherapy history, radiotherapy history, baseline absolute white blood cell count (≥4.0 ×10⁹/L, ＜4.0 ×10⁹/L), baseline absolute neutrophil count (≥2.0 ×10⁹/L, ＜2.0 ×10⁹/L), baseline hemoglobin level (≥110 g/L, ＜110 g/L) and chemotherapy regimens (High risk of FN, intermediate risk of FN). The results showed that patients with baseline hemoglobin of ≥110 g/L and ＜110 g/L had grade 3/4 neutropenia of 5% and 14.8 % respectively (P = 0.017). Patients treated with high FN risk chemotherapy and intermediate FN risk chemotherapy had grade 3/4 neutropenia of 0.9 % and 8.9 % respectively (P = 0.003). No significant difference of incidence of grade 3/4 neutropenia between other subgroups were found. Patients treated with cyclophosphamide and epirubicin (AC, classified as intermediate risk of FN by investigator) had grade 3/4 neutropenia of 15.4 %; Patients treated with cyclophosphamide, docetaxel and epirubicin (TAC, classified as high risk of FN by investigator) had grade 3/4 neutropenia of 14.3 %; Patients treated with docetaxel monotherapy (classified as intermediate risk of FN by investigator) had grade 3/4 neutropenia of 4.9 %. Conclusions: Prophylactic administration of mecapegfilgratim reduced the incidence of grade 3/4 neutropenia/febril neutropenia. Baseline hemoglobin level and specific chemotherapy regimen could be a useful prognostic facots for neutropenic events after administered mecapegfilgrastim.