In this paper, the effects of deformation temperature, strain rate and height reduction on hot deformation behavior and microstructure revolution of as-cast AZ31B magnesiumSalloy were researched in terms of hot compression tests. The results show that the peak stress decreases with the decreasing strain rate and increasing deformation temperature, and the main nucleation mechanism consists of initial grain boundary bulging nucleation, sub-grain rotation nucleation, twinning induced nucleation as well as continuous recrystallization. Moreover, the deformation temperature is below 400_oC, the increase of temperature will beneficial to recrystallization and grain refinement, while as above 400_oC, grain sizeSincreases rapidly. Furthermore, the deformation temperature is equal or lesser than 400_oC, the lower strain rate of 0.1s_-1 is more beneficial to the recrystallized grain refinement, while above 400_oC, medium strain rate of 1s_-1 is more advantageous. In addition, it is found that at higher temperature and lower strain rate, height reduction primarily affects the grain size, and at higher temperature and higher strain rate, height reduction primarily affects the dynamic recrystallization degree.