Abstract: The traditional ultrasonic testing methods for detecting the auto-body spot welded joint especially the faulty welding defects have some application problems such as the bad adaptability and low identification rate. Focused on these problems, the ultrasonic echo characteristics of the spot welding defects were analyzed mainly with the qualitative characteristic parameter analy sis, which can identify the spot welding defects quickly through collecting the standard ultrasonic curves and using peak-value marking algorithm.At last, lots of experiments are conducted to prove that this method is credible and efficient for the spot welding of ordinary steel sheets. The identification rate can reach 95%.