Abstract: Initial ground stress for stability and emulation calculation of underground engineering is very important. To know the values and distribution of initial ground stress, the method of combining surveying in-situ with inversion calculation is usually applied. Particle swarm optimization(PSO)is a good inversion method with characterizing simple algorithm, fast convergence, and easy operation. And FLAC3D is suitable for most mechanical problem of engineering, which is approved and applied widely, and, further more, is easy to develop secondarily with the letin FISH language. Therefore, a program of inverting initial ground stress is compiled by means of studying the theory of PSO and the inversion method of initial ground stress. This program was used to study on inverting the initial ground stresses of Jinping Cascade 2 HydropowerStation Project. The inverting case indicated that this method is simple to implement and the inversion results are accurate.