This paper analyses the relative anomaly of acoustic velocity and study the geology-geophysical trend plane in deciding the unload rebound crack/fracture of dam foundation.

Based on the introduction of basic geological characteristics and relaxed-tension crack feature of a typical bank slope with relaxed-tension crack, the state and trend of bank slope stress are analyzed before and after tension crack formation as well as after water storage of the reservoir.

Through the hydraulic system of several unloading loop performance characteristics of the analysis and comparison,the principle of unloading,use of the notes,as well as the calculation of the power loss and improve the efficiency of the system for the hydraulic system design provide a theoretical basis and practical experience.