Abstract: In order to use ultrasonic pulse reflection method to detect the internal defects of mine machinery thinwalled steel parts, the authors investigated the cause of the failure of the parts through the destructive test and xray test. The technical difficulties in the ultrasonic detection of thinwalled steel members were analyzed. Through the production of reference blocks containing artificial and natural defects, and by comparison method using reference, amplitudes of the artificial defects with different near field distance were relatively accurately set. Fracture test and metallographic analysis were used to determine the defect sizes so as to establish the distance amplitude curve, based on which the distance amplitude curves could be established and hence the inspection of thin walled steel castings by using ultrasonic testing could be realized, solving the problems of its testing without available reference and acceptance standards. The practice has proved that the method can effectively control the quality of thinwalled steel parts to a certain extent, thus solving the problem of ultrasonic inspection of thin walled steel castings.