The mechanical behavior of semi-solid MB15 magnesium alloy was investigated based on an isothermal compression experiment. The concept of thixotropic strength that is the maximum stress borne on the solid skeleton of semi-solid materials is put forward, which is the stress corresponding to the thixotropic point during steady deformation. Effects of some factors, including the heating temperature, strain rate, holding time, solid grain size, grain roundness as well as the intrinsic strength of the materials, on the thixotropic strength were analyzed, and existent conditions of the thixotropic strength was given. The results show that thixotropic strength is decreased with increase the heating temperature, decreasing the strain rate and extending the holding time while is increased with lowering the solid grain size, increasing the grain roundness and intrinsic strength of the material. The thixotropic strength exists when the solid grain in semi-solid materials jointed with each other to constitute the solid skeletons.