The ultrasonic impact stress relief test was carried out on the dissimilar steel welded joint between cutting head toothholder and cylinder body by using different impulse currents and impact time. The results show that after ultrasonic impact treatment, the residual stress relief rate of toe of weld is68% ～122%. When the impact time is constant, the residual stress relief rate will increase as the impulse current increases.When the current is 3.1 A, a large compressive stress is formed on the surface. When the impulse current remains unchanged and the impulse time increases, the decrease of elimination rate is small, and the increase is less than 16%. The optimum current value for ultrasonic impact treatment of the welded joint between the tooth seat of the cutting head and the cylinder is 2.7 ～3.1 A, and the optimum impact time is no more than 250 s.