Abstract: SPCC(steel plate cold common)of cars was welded by Narrow lap resistance seam welder. By using ultrasonic impact treatment technology, the joint of resistance seam welding was treated with the ultrasonic impact. The organization and mechanical properties of treated and untreated seam welded joint were tested and analyzed through tension and compression fatigue test, X-ray diffraction stress measuring instrument, metallographic test and scanning electron microscopy. The results showed that the joint of resistance seam welding, without ultrasonic impact treatment, had big changes in the frequency and cracks under cyclic loading of 5×10⁴ times. The cracks occurred in the weld toe area, and the load direction was a 45-degree angle. The joint of resistance seam welding, with ultrasonic impact treatment, had no changes in the frequency and cracks under cyclic loading of 1.88×10⁶ times. The joint of resistance seam welding of ultrasonic impact treatment was refined in the surface grain and formed the plastic deformation layer, reducing the stress concentration caused by weld defects, such as micro crack of the seam welded joint and the surface of weld toe area. And it is which could improve significantly the fatigue performance of steel plate welded joint.