By means of Friction stir welding (FSW), as-cast AM50 magnesium alloys were welded with travel speed of 50 mm/min and rotating rate of 1200 r/min. The thermal cycle was investigated by thermocouples and a paperless recorder. The results show that phenomenon of dual peaks and hysteresis exist in temperature curves of featured points at the starting part while only hysteresis in the middle part. The finishing part has the highest peak temperature while the starting part corresponds to the lowest one. Featured points for different depths in the same place share the same shape of temperature curves but with different peak temperature. The retaining time over recrystallization temperature at middle part is double that at finishing part.