One of the important geological investigation and research for arc dam is how to select the properly buried depth of the arc abutment. The paper points out that the rock mass unloading degree is one major factor which influences the rock mass quality on the basis of analyzing the geological conditions of the dam site area. Meanwhile the paper describes the rock mass relief features of the both side banks of the valley. The slightly relief load rock mass and no-relief load rock mass can act as the available rock mass for arc abutment. The relief rock mass zones on both sides of the valley are classified according to the results of combining the geological investigation with, seismic prospecting and acoustic prospecting in the adit. Then the properly buried depth of the arc abutment is suggested for designing the high arc dam.