Objective：This study aims to analyze the effect and duration of pegylated recombinant human granulocyte colony-stimulating factor（PEG-rhG-CSF）by studying the dynamic changes of blood routine in breast cancer patients after chemotherapy. Methods：A total of 65 breast cancer patients who needed adjuvant chemotherapy were included. Patients were injected with PEG-rhG-CSF at different time period after the first and subsequent chemotherapy，and the patient’s blood routine results were dynamically monitored. Results：The lowest point in the white blood cell（WBC）count and neutrophil count（NC）occurred around the 10 th day after the first chemotherapy. After the injection of PEG-rhG-CSF，WBC and NC mostly returned to normal levels within 3-5 days. Prophylactic injection of PEG-rhG-CSF in subsequent chemotherapy significantly reduced the incidence of Ⅲ/Ⅳ myelosuppression. Conclusion：Myelosuppression will be the most severe in breast cancer patients on the 10 th day after chemotherapy. Therefore，it is rational to test blood routine on the 10 th day after chemotherapy. Prophylactic injection of PEG-rhG-CSF can reduce the incidence of myelosuppression. PEG-rhG-CSF can still be used in patients with Ⅲ and Ⅳ myelosuppression，and most patients will be cured within 3-5 days.