In this paper, the as-cast AZ31B magnesium alloy after homogenization treatment was taken as the research object, and a Gleeble thermal simulation compression test with deformation temperature of 200~450℃ and strain rate of 0.01~10s-1 was carried out on the Gleeble-3500 thermal simulator. The critical crack strain in the process of thermal compression was determined by high speed photographic technique, and the critical crack damage value was determined by FEM simulation. The results revealed that the classical Freudenthal criterion could well reflect the phenomenon of surface crack initiation and propagation observed by high-speed photographic technique and thermal simulation compression experiment, and the results are consistent with the metallographic observation. Therefore, based on the Freudenthal criterion, a new fracture criterion was established by introducing the Zener-hollomon factor to characterize the effect of deformation temperature and strain rate to the critical fracture damage of magnesium alloy during thermal deformation process. The fracture criterion well indicated the relationship between the critical fracture damage value of magnesium alloy and the stress state, strain, deformation temperature and strain rate, which provided a theoretical support for predicting thermal deformation cracking of as-cast AZ31B magnesium alloy and laid the technical foundation for optimizing the parameters of thermal processing of the alloy.