Abstract: The feasibility research of the field is the vital task of nuclear power engineering site selection.As for the bad weathered cystid existed in the field of Tianwan Nuclear Power project investigation,the current study,taking the measure of increased drilling and analysis of sample on the basis of the original exploration scheme,and combining the means of shallow layer seismic prospecting,sound wave logging,point load testing and laboratory testing,has found out the distributing range of the weathered cystid,discussed the reason for its formation,and appraised the intactness of rock mass in the weathered cystid and the weathered grade of the rock,thus has obtained the mechanics index and the dynamic parameters which demonstrate the engineering properties of weathered cystid.The study has then analyzed the influence of the weathered cystid toward the construction of nuclear power project,which have provided a scientific basis for the foundation design of nuclear power project.