Abstract: Using advanced ultrasonic detection equipment, the highest wave of the dynamic envelope as derived by the frame-by-frame analysis of the endpoints is selected to measure the crack depth detected by magnetic powder in regular maintenance of the catheter rack of the sea oil platform and also to measure the shallow cracks with a depth of 2. 0 to 3. 2 mm. Test result show that this technology can evaluate the depth of crack extension more comprehensively and accurately, and it can also completely record and collect data. Its operation is simple and easy to implement, it can also be used for higher-precision bathymetrics of shallow defects under various conditions.