Abstract: In this paper, by the ultrasonic physical forward simulation, the reflection dynamic characteristics on the single entry type for diffrent void dimensions are analyzed, the relationships of the reflection amplitude with the size of the void to wavelength ratio are revealed, and 3 meter minimum resolving power in reflection for discovering the void are found out. Secondly, the type of large mining area is simplified, and its kinematic and dynamic characteristics of reflection are expounded. Finally, the reflected longitudinal and transverse wave for locating the multiple entry type are discussed.