The TiB2/AZ31 magnesium based composite was fabricated by the in-situ synthesis-semisolid stirring technique, and its microstructure and mechanical property were studied. The results show that hot extrusion not only significantly refines the microstructure of the alloy, but also effectively improves the uniformity of TiB2 particles distribution. Compared with as-cast AZ31 magnesium alloy, the hardness and tensile strength of as-cast TiB2/AZ31 magnesium based composites increase to some extent (unobvious), but the elongation decreases. However, after extrusion, the hardness and the tensile strength of the as-extruded TiB2/AZ31 magnesium based composite increase by 126.2% and 98.8%, reaching 950 MPa and 322 MPa, respectively, and the elongation is also higher. Adding of TiB2 particles to AZ31 alloy and hot extrusion are effective ways to improve the wear resistance of AZ31 magnesium alloy.