To investigate the effect of SiCp on the evolution of the microstructure and properties of an AZ91 magnesium alloy during electric pulse treatment (EPT), a SiCp/AZ91 composite was prepared by adding 1%(volume fraction) micron SiCp to an AZ91 magnesium alloy. The microstructure of the AZ91 alloy and SiCp/AZ91 composite was refined by low temperature extrusion and electric pulse treatment. The microstructure evolution was analysed by optical microscope. The mechanical properties of the alloys and composites were tested at room temperature. The results show that the composite with reinforced phase particles has a greater dislocation density and greater deformation storage energy after extrusion compared with AZ91 alloy, thus promoting static recrystallization during electric pulse treatment. The yield strength and tensile strength of the AZ91 alloy and composite after pulse treatment are 320 and 450 MPa, and 380 and 454 MPa, respectively. Due to the stress concentration at the interface between the SiCp and the magnesium matrix, the tensile strength of the composite lightly increases.