Based on 1.8 mm thickness 304 and 430 dissimilar stainless steel sheets,the three-dimensional finite element model for heat source of friction stir welding is established by using numerical simulation software COMSOL. The temperature field distribution of friction stir butt welding of 304 and 430 dissimilar stainless is analyzed, and the effects of the rotation speed and welding speed of FSW tool on peak temperature are studied. The results show that peak temperature of the heat source center rises when the rotation speed of FSW tool increases but decreases when the welding speed increases. When rotation speed is 800 r/min and welding speed is 15 mm/min,the peak temperature increases rapidly at first and then reach the steady value of about 1 040 ℃ which appears in the rear side close to heat source. The expansion of temperature in 430 backside area is larger than taht in 304 advancing area,showing a more uniform temperature distribution. The predicted temperature filed based on finite element method shows agreement with the measured value.