According to the data of the hydroelectric boreholes in deep-cutting valleys in west of china, it can be concluded that the phenomena of unloading and relaxation in the deep-cutting river valley bottom are widely spread. The average thickness of the unloading and relaxation belts ranges from 10 m to 20 m, the characters of these zones are low-weathered, cracks developed, low rock strength, low completeness and high permeability, etc. The research found out the rocks in the valley bottom experienced a series of processes to turn into the loading release belts which are stress concentration in the valley bottom-deformation and broken of the rocks-rocks on the surface become loosened-secondary cracks developed-long-term inter-reacted between rocks and water-pressure release belts formed. By lab physical simulation of the processes which is described before, it can be seen directly the whole process of how pressure release belts are formed.