Abstract: Stress concentration factors have a significant effect on the load capacity of overlap joint.The influence of joint geometric parameters on stress concentration factors of bilateral welded undermatching overlap joints were studied based on the finite element method,and the stress concentration equation was founded based on regression analysis method.The result shows that the lap length plays the most important role in stress concentration factors.The stress concentration factors at weld root increase with the lap length increasing,weld width decreasing and weld toe smooth transition radius decreasing.The stress concentration factors at weld toe increase with the lap length and weld width increasing.Weld toe with smooth transition radius has little effect on concentration factors at weld toe.The stress concentration equation has a good agreement with the finite element result.