The surface cracks of explosive parts have important influence on its mechanical properties and detonation properties and can remove the surface cracks by machining margin, and the accuracy of crack depth is the key. Based on the basic principle of ultrasonic wave edge peak echo, a quantitative detection method of surface crack depth for HMX based polymer bonded explosive (PBX) was established. The simulation test blocks with different-depth cracks were designed and manufactured. The quantitative precision of crack depth of ultrasonic edge peak echo method was checked. A better experiment repeatability was found. The different value between measured depth and actual one is less than 1.0 mm. The detection results of crack depth of explosive obtained by ultrasonic testing and micro focus X-μCT imaging technique were compared. The crack height results have good consistency, indicating that the edge peak echo method for measuring crack depth is reliable. The ultrasonic testing error sources are analyzed. Results show that the crack depths detected by the edge peak echo method are close to actual crack depths, with the error within ±1.0 mm.