The effect of micro-Mn addition on the microstructure and mechanical properties of Mg alloys was analyzed. Electron back scattered diffraction, slip traces analysis, transmission electron microscopy and visco-plastic self-consistent polycrystal constitutive modeling were performed to investigate the deformation mechanisms in the tensile test. The results show that the average grain size of Mg-Mn alloys decreases from 28.30 to 5.10 μm with Mn addition. Furthermore, with the increment of Mn content, the ductility of the as-extruded samples improves from 14.33% to 20.33%, and the yield strength increases from 84 to 180 MPa. The reason for this simultaneous improvement is that the critical resolved shear stress gap between basal slip and non-basal slip decreases from 173 to 115 MPa, which leads to the increase of non-basal slip activity. The adjustment of slip system difference by microalloying elements provides a way to develop Mg alloys with high strength and ductility.