Stress distribution and failure behavior of laser transmission welding parts are studied by means of simulation and experiment during the tensile process. The welding parts of PA66 are chosen as the research object and the numerical simulation model of the tensile experiment is established. The tensile deformation and the corresponding load-displacement curves of welding parts are simulated and compared with the results of tensile experiments. Analysis of the shear and Von Mises stress distribution of welding parts during the process of tensile experiment is carried out. The failure behavior of tensile specimen can be studied from the aspect of shear and tensile failure. The numerical simulation model can well predict the tensile deformation of the welding parts. The maximum shear stress occurs in the vicinity of the four corners of a rectangular weld area near the weld interface, which are also the starting positions of the shear failure. Because the maximum shear stress is much less than the shear strength of PA66, the tensile shear failure is less likely to occur. The predicted tensile failure and the failure position of welding parts by numerical simulation are consistent with the experimental results. The accuracy of numerical simulation method for analyzing and predicting the tensile failure behavior of the welding parts and the reliability of the numerical simulation model of tensile process are verified.