Abstract: The residual stress has great influence on welding quality and it can result in the formation of the cracks in the weld during laser deep penetration welding. In this paper, the heat source model was established through the analysis for the metallographic structure of the weld of fine grained steel, and the extraction of chemical composition of fine grain steel was conducted by the PDA-5500S SHIMADZU optical emissions spectrometers. Based on Gibbs function, the coupling constitutive relation between temperature and material flow stress was established. Furthermore, the real-time residual stress was achieved by setting trace points in finite element analysis model based on the analysis of the experimental data for the cross-section of weld bead by different welding parameters. The results show that when the real-time residual stress is higher than the material flow stress at the same temperature, the cracks will occur in the weld bead.The real-time residual stress value can be decreased through the reasonable increase of welding speed, and it can improve the quality of laser deep penetration welding.