For the stability analysis of underground cavern, initial stress regression analysis is a very important first step. The existing initial stress regression methods, such as function method, displacement back analysis method and boundary method, are not easy to operate. Their accuracy is not high. This paper takes the Huangdaowater sealed underground oil cavern as an example. The relationship between the principal stress and the vertical self-weight stress is obtained, according to the measured initial in-situ stress. On the basis of the numerical simulation calculated results of self-weight stress, the horizontal principal stress convert to the normal stress and shear stress of model coordinates by coordinate transformation. Finally, based on the FISH language of FLAC3D,read the normal stress and shear stress obtained in the previous step, and calculate to convergence. Then the initial stress field is generated directly. The calculated results are compared with the measured results in the field. The relative error is less than 10%. This method is simple and effective.