OBJECTIVE： To investigate clinical efficacy and safety of trastuzumab combined with neoadjuvant chemotherapy on breast cancer， and discuss its effects on the expression of HER2 protein， B7 homolog 1 （B7-H1） and IL-2. METHODS： 62 patients with breast cancer confirmed by pathological biopsy， whose test results of immunohistochemistry combined with fluorescence in situ hybridization （FISH） was overexpression of HER2， were selected as the research object. They were randomly divided into observation group and control group with 31 cases in each group. Control group was given carboplatin combined with docetaxel neoadjuvant chemotherapy before surgery. Observation group was additionally given Trastuzumab for injection intravenously 4 mg/kg in the first week， 2 mg/kg in second-eighteenth week， once a week， for consecutive 18 weeks. Modified radical mastectomy was conducted 2 weeks after chemotherapy. Clinical efficacy and the occurrence of ADR were observed in 2 groups. The expression of HER2 protein， B7-H1 and IL-2 in tumor tissues were compared between 2 groups before and after treatment. With 1-year follow-up， tumor recurrence rates， metastasis rates and survival rates were recorded in 2 groups. RESULTS： After treatment， total effective rate of observation group was 83.87％， which was significantly higher than that of control group （58.06％）， with statistical significance （P＜0.05）； the test result of HER2 protein in lesion tissues was non-overexpression in 19 cases （61.29％） in observation group which was significantly higher than that of control group （11 cases， 35.48％）， with statistical significance （P＜0.05）； the positive expression rates of B7-H1 and IL-2 in lesion tissues of observation group were significantly lower than those before treatment and control group after treatment， with statistical significance （P＜0.05）. There was no statistical significance in the incidence of ADR between 2 groups （P＞0.05）. During 1-year follow-up period， except that there was no statistically significant difference in survival rate between 2 groups （P＞0.05）， the recurrence rate and metastasis rate of observation group were significantly lower than those of control group， with statistical significance （P＜0.05）. CONCLUSIONS： The application of trastuzumab combined with neoadjuvant chemotherapy before breast cancer modified radical mastectomy can effectively improve the expression levels of HER2 protein， B7-H1 and IL-2 in lesion tissue of patients with breast cancer， promote clinical efficacy with good safety and low risk of tumor recurrence and metastasis after operation.