Objective To investigate the effect of miR-185-5p on the proliferation, invasion, and migration of mouse breast cancer PY8119 cells. Methods miR-185-5p with significantly down-regulated expression in breast cancer was obtained through the preliminary research class; NCBI query human and mouse miR-185-5p gene sequences; EdU proliferation, Transwell invasion, scratch healing and flow cytometry were used to detect the effects of overexpression and knockdown of miR-185-5p on the proliferation, invasion, migration and apoptosis of PY8119 cells. The subcutaneous tumor model and lung metastasis model were established in C57BL/6 mice, and the changes of tumor proliferation ability and lung metastasis in vivo by overexpression of miR-185-5p were observed. Results EdU proliferation, Transwell invasion, scratch healing and flow cytometry assays 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. The in vivo tumorigenesis experiment in C57BL/6 mice showed that overexpression of miR-185-5p slowed down tumor growth. Lung metastasis analysis showed that overexpression of miR-185-5p inhibited lung metastasis in C57BL/6 mice. Conclusions miR-185-5p as a tumor suppressor gene inhibits the proliferation, invasion, and migration of mouse breast cancer PY8119 cells in vitro and in vivo.