The accurate determination of initial geo-stress field is the important premise of numerical simulation to the excavation of underground powerhouse. It is usually measured to get the value of initial geo-stress field. But it can not meet the demand of construction by this way. So it is very important to seek the methods that simulates initial geo-stress field according to several measured points. The FEM analysis-linear regression is usually used to simulate the initial geo-stress field. In this paper, the development of back-analysis to initial geo-stress field and the method of FEM analysis-linear regression are briefly introduced, and the initial geo-stress of an underground powerhouse is back-ananlysised. The results show that the back-analysis is concordance with the measured data.