Abstract: The instantaneous velocity of stirring tool during friction stir welding (FSW) process was analyzed to build a heat source model of FSW, which took into account the heat difference near the weld line. The temperature and stress fields of 2024 aluminum alloy during friction stir welding were numerically simulated by using 3D finite element method. The results show that the asymmetry of temperature and stress fields was not obvious because the moving velocity was much smaller than the rotating velocity of stirring tool, but this asymmetry would be enhanced with increasing the moving velocity. The temperature at the weld center was lower than that at the tool edge during welding. The stresses were compressive at the weld front and on two sides, but tensile at the weld rear. After welding, the transverse and longitudinal residual stresses were large tensile stress within the stirring tool, which gradually became smaller along the depth. Far from the weld line, the transverse and longitudinal residual stresses were very small. The residual stresses by finite element simulation were compared to those by short wavelength X-ray stress measurement; two results were basically identical in trend.