Abstract: This study aims to investigate the effect of transmembrane protein angiotensin converting enzyme 2 (ACE2) on the prognosis of breast cancer and its potential mechanism.Public databases were used to analyze ACE2 expression and its relationship with clinicopathological features and prognosis of breast cancer patients, combined with in vitro experiments to analyze the mechanism of action and immune relevance of ACE2 in breast cancer.Results showed that the expression of ACE2 in breast cancer tissues was significantly lower than that in normal breast tissues, and that its expression was negatively correlated with age, M stage and N1mi stage of breast cancer patients (P < 0.05).Patients with Luminal type breast cancer with high ACE2 expression had poor prognosis, while in the triple-negative breast cancer (TNBC) subtype, ACE2 showed different prognostic significance.In addition, ACE2 is closely associated with the metabolic and immune microenvironment of tumor tissue.In vitro experiments have shown that ACE2 is lowly expressed in MDA-MB-231 cells and may inhibit cell progress by downregulating matrix metalloproteinase 2（MMP2）.The results suggest that the low expression of ACE2 in breast cancer is closely associated with patient prognosis as well as metabolic and immune microenvironment, and that ACE2 may inhibit TNBC cell progress through the MMP2 pathway.