Abstract: In order to study the effect of shear deformation on the microstructure and mechanical properties of the pre-twinned AZ31 magnesium alloy, the pre-twinned samples with a strain level of 5% were subjected to different passes of equal channel angular pressing(ECAP)at 250℃.Subsequently, the microstructure observation and room-temperature uniaxial tensile test were carried out for the samples in different states. The results show that the microstructure of the samples is obviously refined compared with the commercial as-extruded alloy, and the ductility is improved greatly after ECAP deformation of the pre-twinned sample. And with the increase of passes, the grains are finer and more uniform, and the ductility is improved significantly. Among them, the room-temperature fracture elongation of the 4P sample is about 163%higher than that of the extruded alloy, indicating that the introduction of shear deformation can effectively improve the room-temperature plasticity of Mg alloy after pre-twinning.