Abstract: The welding residual stress field in a certain type of plate-fin EGR cooler was simulated by numerical analysis method. The cause of leakage failure in the performance testing of the EGR cooler was investigated from the view point of welding residual stress. The simulation results showed that the peak welding residual tensile stress of the EGR cooler occurred at the tube-plate joint position near the round corners of the flat tubes,which coincided with the location where the macroscopic cracks that led to the leakage appeared in the EGR cooler. Optimizing the fusion welding sequence could decrease the magnitude of peak welding residual tensile stress in plate-fin EGR cooler significantly.