Abstract: The existing ultrasonic testing system has a large error in evaluating the nugget diameter of resistance spot welding joints. In order to solve this problem,the quantitative testing method of nugget diameter for spot welding was proposed, which was based on phased array ultrasonic probe and characteristic signal analysis technology. The influence law of surface indentation on the single array ultrasonic signal was emphatically studied. The edge identification method of nugget based on array ultrasonic signal was revealed. Then the accuracy of testing method of nugget diameter was experimentally verified. The results show that results of the spot welding nugget diameter obtained by ultrasonic testing were highly consistent with that of metallographic measurement results,which is highly reliable and accurate.