Based on the measured initial stress, topography, geomorphology, and geological structure such as fault, a 3D FEM model was established for the multivariable regression back analysis of initial stress field in the underground powerhouse area of Changheba Hydropower Station. The results show that the FEM regression stress values are in good agreement with the measured values of the rock mass in the underground chamber area, and the regression method is effective for the back analysis; the horizontal stress is larger than the vertical stress, and the initial stress increases with the horizontal and vertical depth; and stress concentration appears near the fault, which should be paid attention for.