Hot deformation behavior and microstructural evolution of the as-cast Mg-3Sn-1Mn-1La alloy were investigated by isothermal compression at temperature of 200-450℃ and strain rate of 0.001-1.0s-1. The flow stress increases obviously with decreasing temperature and increasing strain rate. The recrystallized grain size increases with increasing temperature and decreasing strain rate. At lower deformation temperatures, continuous dynamic recrystallization is the main mechanism of DRX. However, discontinuous dynamic recrystallization becomes the predominant operating mechanism of DRX at high deformation temperature. The effects of friction and deformation heating on flow stress were analyzed and corrected. The results show that deformation heating has a significant influence on the flow stress at higher strain rates and lower temperatures, while the frictional effect is slight. Based on the experimental results, a strain-compensated Arrhenius-type equation was developed. Comparison of the experimental data with the calculated flow stress indicates that the developed constitutive equations can adequately describe the hot deformation behavior of the experimental alloy.