In this paper, the hot compression tests were carried out, the true stress-strain curves were obtained during AZ31B cast rod deformation, and the effects of deformation temperatures and strain rates on flow stress were analyzed. It was found that the peak strain increased with the increasing strain rate and decreasing deformation temperature, and it is advantageous for dynamic recovery and recrystallization to reduce strain rate and properly raise deformation temperature. Moreover, the flow stress prediction model, which was established by means of multiple regression analysis can illustrate the strain-sensitivity of flow stress. Furthermore, it is investigated that the correlation?coefficient of predicted and experimental values is up to 0.9926, indicating that this model is more accurate in terms of predicting the flow stress of as-cast AZ31B magnesium?alloy and can better?describe its flow behavior during hot deformation.