In this research, hot deformation behavior of LZ91 Mg alloy was explored. Then, the active energy and the constitutive equation were evaluated and obtained. The results show that the compressive process of LZ91 alloy is a typical rheological process. The thermal activation energy of LZ91 alloy in temperature range of 260-380 degrees C is 172.2kJ/mol. The constitutive equation of it in the hyperbolic-sine function can be expressed as:. As deformation temperatures rise, the mount of beta phase was reduced while alpha phase was precipitated from beta phase due to Li break away from beta phase. The reason of grain refinement and main softening mechanism can be credited to dynamic recrystallization.