The invention relates to a detection method for a stainless steel composite welding gap. The detection method comprises the following steps of (1) debugging a K value and front-edge distance of an ultrasonic probe; (2) fabricating an ultrasonic DAC curve; and (3) verifying a scanning result of an ultrasonic device. By the technology, flaw detection is performed from a composite layer surface of acomposite steel plate by an ultrasonic fault detector, the accuracy and the quality of composite welding gap detection by ultrasonic wave can be ensured, the problems of ineffectiveness of a traditional ultrasonic detection method, a large amount of radiographic inspection cost and labor waste are prevented, and a security risk of a detector during operation is reduced.