Abstract: Based on hydrogeology and engineering geology, the storage capacity of a depression on mountain top in Chengshan is studied. The depression is estimated to have the capacity to hold 3.85 million cubic meters with a leakage of 0.96 million cubic meters in 24 hours. As result, the real storage capacity for power is 2.90 million cubic meters, which satisfies concerned departments with 2 to 3 million cubic meters to be held.