Abstract: The higher order harmonic component of nonlinear ultrasonic is obtained by the general ultrasonic testing device with a higher pass filter,and the higher order harmonic wave is adopted into the tiny flaw detection and the ultrasonic imaging of spot welds.The higher order harmonic wave has excellent sensitivity to the tiny flaw and higher resolution in the time domain than conventional ultrasonic wave.Thus,the size and the position of tiny flaw are easier to determine,though the thickness of spot weld is very small.Besides,the nonlinear ultrasonic imaging technique based on the higher order harmonic wave is also presented in the research,and the nonlinear ultrasonic imaging is more effective to investigate the inner constitution of spot weld.The nonlinear ultrasonic has a higher testing effectiveness,and the testing ability of ultrasonic system can be developed.