Abstract：The concept of dynamic unloading of initial stress of rock mass in the excavation process of slope and underground powerhouse is presented. Taking the excavation of underground powerhouse of Longtan hydropower station as an example，the influences on rock mass induced by blasting load and dynamic unloading of initial stress are compared. The results show that the high dynamic unloading of initial stress during rock excavation may be an important factor inducing looseness of rock mass. Damage in neighboring field of hole is mainly caused by blasting load，but damage in far field of hole may be induced by dynamic unloading of initial stress during rock excavation，i.e.，the higher unloading rate of initial stress is，the lager the loose zone of surrounding rock mass is.
