Abstract: E'mei mountain basalt is one kind of rock mass of hydropower engineering in our country.We determine the surface of dam foundation based on weathering degree of rock mass.Weathering of basalt controlled by rock mass structure,start on surface of various rock mass structures,and developed into interior gradually.Taking one hydropower station of the southwest China an example in this paper,the authors study on the weathering of apparent characteristics,mineral characteristics and chemical characteristics of E'mei mountain basalt on dam-site.It suggested that weathering of basalt mainly is physical weathering,and chemical weathering is not obvious,and fault,interlaminar and intrastratal disturbed belt are not obvious effect in chemical weathering on the basalt.Judging from the boundary of rock mass,weathering on the site is mainly depend on apparent characteristics of rock mass,finally the paper provide reference for judging weathering boundary on the site.