Due to the highly-localized heating and non-uniform cooling in spot laser keyhole welding, complex residual stresses are often found in final welds, which can cause detrimental effects on mechanical properties of the welds and result in problems like cracking and distortion. Since the formation of thermal stress is tightly related with the heating, melting, solidification, and cooling processes in welding, accurate analysis of the transient thermal transport phenomenon in welding is critical to predict the thermal stress information in final welds. In this study, a comprehensive thermal model analyzing the heating, melting, solidification, cooling processes in spot laser keyhole welding is integrated with a mechanical model to accurately predict the thermal stress evolutions in laser welding and the resultant residual stresses in final welds as well. High compressive residual stress was found on the top surface of the workpiece in welding which causes rough surface (bumps) of the final weld. Residual tensile stress in some locations in the final weld was found to be higher than the critical level which indicates the potential thermal cracking occurrence. The proposed models can be used to further study the thermal stress formation mechanisms in laser welding and to provide a cost-efficient way to optimize the welding operations.