Abstract: The left abutment slope of a hydropower station on Jinsha River is a strip-type ridge formed by cutting and surrounding of two rivers, and the unloading relaxation of slope rock mass is intense, in a certain depth the rock mass cracked with maxium widest of 15 cm. On the basis of field investigation and combining with the factors such as the landform, stratum lithology, structure development characteristics and river valley ground stress, we carry out comprehensive analysis on the unloading relaxation mechanism. The results show that isolated strip-type ridge with 3 free surfaces is formed by the cutting of Jinsha River and its tributary, and the vertical height of ridge is 350m. The unloading depth of 112m is formed by quick and strong stress release, tension fracturing and toppling over of steep and reverse rock mass, and the continuous shearing and dislocation with time.