On the basis of the field investigation and the indoor analysis， the cause of typically deep differential weathering formation and distribution law existing in some power station dam area of southwest China is analysed. Its developing distribution is located in the inside of the lightly weathered, fresh rock, mainly controlled by lithology and fracture ,and its causes are also closely related to the formation and evolution of the river valley and bank slope. Deep differential weathering would not directly affect the stability of the rock, but it contributes the further development of epigenetic reformation of rock mass, resulting in the loosening, deformation, destruction, instability along the weak structural face of the rock.