To further explore the mechanism of ulinastatin in the proliferation and apoptosis of breast cancer cells, the expressions of ERK and FOXO3, key factors in the ERK signaling pathway, were examined at the gene and protein levels in this study, and the results imply that ulinastatin markedly suppresses the expressions of ERK and FOXO3, and the effect is more significant after the use of the ERK inhibitor. Here, FOXO3 is linked to breast cancer.