Object To investigate the effect of miR-185-5p on the proliferation, invasion and migration of mouse breast cancer PY8119 cells. Methods Mirna: miR-185-5p with significantly down-regulated expression in breast cancer was obtained by previous research group. The sequence of miR-185-5p gene from human and mouse was consistent by NCBI. The effects of overexpression and knockdown of miR-185-5p on the proliferation of PY8119 cells were detected by EdU proliferation experiment. The effects of overexpression and knockdown of miR-185-5p on the invasion ability of PY8119 cells were detected by Transwell invasion experiment. The effects of overexpression and knockdown of miR-185-5p on the migration ability of PY8119 cells were detected by scratch healing experiment. The effects of overexpression and knockdown of miR-185-5p on the apoptosis of PY8119 cells were detected by flow cytometry. The effect of overexpression of miR-185-5p on tumor proliferation in vivo was detected by C57BL/6 mouse subcutaneous tumor model. Lung metastasis of mice overexpressing miR-185-5p was observed by tail vein injection. Results The results of EdU proliferation experiment, Transwell invasion experiment, scratch healing experiment and flow cytometry showed that knocking down miR-185-5p significantly enhanced the proliferation, invasion and migration of PY8119 cells and inhibited apoptosis. Overexpression of miR-185-5p inhibited the proliferation, invasion and migration of PY8119 cells and promoted apoptosis. In vivo tumorigenesis experiment of C57BL/6 mice showed that overexpression of miR-185-5p slowed down the tumor growth rate in C57BL/6 mice. Lung metastasis experiment showed that overexpression of miR-185-5p inhibited the lung metastasis of C57BL/6 mice. Conclusion miR-185-5p can inhibit the proliferation, invasion and migration of mouse breast cancer PY8119 cells in vitro and in vivo as a tumor suppressor gene.