The investigation of the plunge stage of the tool during friction stir welding (FSW) is the basis of understanding the nature of FSW, which is especially important for friction stir spot welding. In this study, the plunge stage of FSW tool was numerically examined by dynamic explicit finite element method with more attention to the temperature evolution. It was found that the high temperature region around the stirred zone extended with increasing the rotation speed. The heat affected zone expanded with the increase of inserting time, but the highest temperature around the stirred zone changed little. The range of the stirred zone changed little with the variations of the rotation speed and inserting time. The simulation result on the temperature field was in comparable agreement with the experimental one.