Abstract: The anti-slip stability of high arc dam shoulders depends on the development of lateral crack planes boundaries. In many cases, these cracks consist of joints in random distribution. So it is of very important engineering significance to find out the characteristics of the joints for stability analysis and design of dam shoulders. In this paper, taking a designed high arc dam located in southwestern China as an example, the authors make a systematic quantitative description of the joints system at the damsite. Several tens thousands of data about the joints were collected in extensive field investigation. These data were analyzed carefully and all features, such us joints distribution, geometry and structures were picked up. Based on these features, a distribution model was quantitatively set up and an engineering of zoning of joints is suggested, which gave a clear and visual expression of the joints development and the state of the lateral crack planes, especially the general exsistence of joints for anti-slip stability of the dam shoulder. Finally, the condition of anti-slip stability of the dam shoulders was assessed in terms of the joints model presented in this paper.