Flow stress and microstructures evolution of AM50 alloy during upsetting forging tests were investigated in this paper. In upsetting forging deformation, cylindrical billets of AM50 alloy as-cast were compressed in the temperature range of 250°C to 350°C and strain rate range of 1.1×10-3s-1 to 2.8×10-2s-1. Under the deformed conditions, the true stress-true strain curves indicated that the true stress increased quickly at the initial time, then stayed at certain value for a short duration and increased continuously at lower temperature and high strain rate. It should be noted that the true stress kept the dynamic balance with the true strain at higher temperature and lower strain rate that the true stress obeyed the regulation as the same as the ordinary materials. The peak stress at all conditions increased with the strain rate increasing and the forging temperature decreasing. A great many recrystallized grains and dislocations were observed in the deformed specimens of AM50 alloy. The grain size of the deformed specimens decreased with decreasing forging temperature and increasing the deformation.