Abstract: Determining t'he field of initial stress of rock masses, available for the engineering practice, is an urgent problem at home and abroad. Using the measured results of stress of rock masses in-situ, authors of this paper have solved the initial stress field of rock masses by applying the mathematical models of finite element in the regression analysis. And the mathematical models are built upon the geological and geographical research data. Considering the probable factors causing the stresses in-situ, a linear expression between the stresses and these factors has been established. The regression equation is set up by using probabilistic method. And by the multi-factor regression procedure the significant factors are discriminated and the insignificant ones are rejected. The method, assuring the solution to be unique and reasonable, should provide an available three dimensional initial stress field of rock masses for engineering design of structures.