Abstract: The rock mass unloading is one of engineering-geological problems in construction of hydroelectric projects. This paper introduces the background and formation mechanism for the rock mass unloading, the common characteristics of unloded rockmass and several methods for discriminating the unloaded zone. Considering the numbers and opening degree of joints in rock mass gradually reducing from the surface to depth, basing on field investigation data of Indo sinian granite on a high slope of hydroelectric station on the upstream of Yellow River, the rock mass unloading zone is discriminated in accordance with amounts and opening degree of joints in combination with rock mass structure and its engineering properties. This result provides a reference for other slope engineering.