The present invention based on non-destructive ultrasonic detection principle and image processing method. Water soaked focusing probe is used in pointwise scanning on the switch combining surface and the ultrasonic echo signal of each point is sampled to constitute the ultrasonic scanning image of the combining surface. Proper threshold is selected in image processing to separate combined area and un-combined area in the ultrasonic image, and interface combination rate is calculated. The detection system consists of ultrasonic signal generator, probe, 3D precise scanning platform, image detection card, stepped motor driving card, stepped motor driving power source, industrial control computer and system software. The present invention is automatic, and has accurate, efficient and reliable non-destructive detection of switch contact combining quality.