Abstract: Based on SYSWELD finite element analysis software, the transient temperature distribution, residual stress and deformation of Q345/316L dissimilar steels were numerically simulated, and the simulation results were validated by the experimental method. The experimental results show a good agreement with the numerical simulation results, which proved the reliability of the dissimilar steel welding of SYSWELD simulation. The results show that the welding temperature field of dissimilar steel was asymmetric, and the Q345 side had a wider range of high temperature regions. Both the transverse and longitudinal residual stresses were in the shape of cap in the direction of the weld and there was a maximum residual stress in the middle of the weld. In the middle of perpendicular welding cross section, the longitudinal residual stress and transverse residual stress were not continuous weld and weld near the central section of the weld. There was a large stress gradient and the stress state was complex. The maximum residual stress appeared in Q345 side of the fusion line. The simulation results under different heat input show that, under the premise of ensuring the quality of welded joints, it was best to use a small heat in welding process.