Circadian locomotor output cycles kaput (Clock) gene is a core gene in the circadian rhythm system that is involved in cancer cell proliferation. However, the molecular mechanism of Clock gene participate in the cancer cell proliferation is unclear. Previous studies demonstrated that cell proliferation could be regulated by the canonical Wnt pathway (also known as the Wnt/beta-catenin pathway), and the Wnt/beta-catenin pathway had a relation with the circadian system. To investigate whether the Clock gene affects the proliferation of breast cancer cell by regulating the expression of beta-catenin, we knocked down the Clock expression of mouse breast cancer cells (4T1) by RNA interference. Then detected their proliferation rates using CCK8 assay and the expression of the beta-catenin gene by real-time PCR and Western blot. The results showed that the proliferation of the Clock knocked down 4T1 cells is slower than the control. The expression level of beta-catenin of these 4T1 cells is reduced. Our study showed that Clock gene knocked down inhibiting the proliferation of the 4T1 cells, probably by suppressing the expression of beta-catenin.