Abstract: This paper is about the geological investigation results at a proposed large hydropower station site. The sheet joints with gentle and medium dips are developed in the superficial slope and manifested as disjunctive distribution of joint concentration areas and joint sparse areas. They control the rock mass structure and stability of high slope. This finding is based on massive field investigation. The paper summarizes and analyzes the basic geological characteristics and distribution characteristics of the sheet joints at the dam site, and explains the joints formation mechanism and development model. The results of the study show that:(1)Sheet joints paralleled to bank slope belong to tension joints-system due to down-cutting of valley and displacement of stress.(2)The formation of bank slope is a result of repeated down-cutting of valley. After down-cutting, the angle of slope becomes steeper. That's mean dip angle of sheet joints formed in the late is steeper than that in the early stage. When the valley generates down-cutting and lateral erosion, the convex bank slope is beneficial to save earlier gentle joints. This is the reason that both gentle dip joints and moderate dip joints are available in the upper-middle part of the convex bank. (3)The canyon is formed by intermittent rapid down-cutting. During relative stable stage, sheet joints are developed in bank slope. During rapid down-cutting stage, vertical unloading mainly develops in the valley bottom. Thus in the bank slope, the disjunctive distribution of joint concentration areas and joint sparse areas are observed, which corresponds to relative stable stage and rapid down-cutting stage, respectively.