Based on the two-stage forming technology, the casting AZ31 Magnesium alloy bar was forged into cylindrical straight inner gear between the temperature 250°C-400°C. At 250°C, the teeth of the inner gear are almost formed. But there are some cyclic cracks on the surface of the sample. When improving the temperature above 300°C, the surface quality of the sample has greatly improved. According to the result of this experiment, the best temperature range for forging AZ31 magnesium gear is 280°C to 380°C.The forming load gradually reduced with the temperature improved. At 250°C, the forming load is 93t. At the 400°C, the forming load reduces to 80t.The initial grain size of AZ31 magnesium alloy bar is 22μm. The microstructure evolution during the warm deformation was observed by optical microscopy (OM). It is demonstrated that the grain refinement happened during the deformation process.