While the influence of orientation of structure plane on parameters of rock mass is neglected in the traditional GSI system. In this paper,as a case of cavern Ⅱ in the Jurong Pumped-storage Hydroplant Project,the influence coefficient between the distribution of structure plane and surface condition rating( SCR) of surrounding rock is calculated based on the space relation of structure plane and axis of cavern; the statistical method of SCR of surrounding rock is modified,and the experiential estimates of equivalent physical mechanic parameters of surrounding rock mass are acquired.The investigations show that the calculative errors of equivalent deformation modulus and cohesive strength in surrounding rock based on modified GSI system decreased by about 22. 3%,8. 3% respectively.