Compound Opening Arrow Mixture (COAM) has demonstrated therapeutic effects in patients with breast cancer. We explored the underlying molecular mechanisms of COAM using a mouse model of breast cancer. Luciferase-labeled 4T1-Luc2 cells were inoculated into the breast pad of BALB/c-nu mice, which were divided into model group (saline), COAM (6 g/ml high-dose, 3 g/ml medium-dose, and 1.5 g/ml low-dose) groups, and low-molecular-weight heparin (LMWH, 1500 U/Kg) group. The number and distribution of 4T1-luc2 tumors were measured by an in vivo imaging system. Tumor cell apoptosis was measured through TUNEL and quantitating the expression of Caspase-3 mRNA and protein. Compared with the model group, in vivo tumor growth was lower in the LMWH- and COAM-treated groups. Tumor apoptosis was time-dependent and dose-dependent, as shown by a higher TUNEL apoptotic index and higher Caspase-3 mRNA and Caspase-3/cleaved-Caspase-3 proteins levels on the 14th day than the 7th day. The COAM high-dose group had the highest apoptotic index and the most activation of Caspase-3. Collectively, COAM significantly inhibits the growth of 4T1-luc2 breast cancer in mice and induces tumor apoptosis by activating Caspase-3, which provides a preliminary explanation of therapeutic effects of COAM.