Initial geostress is one of the most important factors influencing stability of rock engineering and is basic index necessary for design of geotechnical engineering. On the basis of engineering geological conditions and measured geostress data of Jinping Hydropower Project, grey control system theory and finite element method are applied to analyze the initial geostress of dam region and the optimum regression coefficient can be derived. Through comparison between computed and measured geostress values of measuring points, it has been found that the two are similar in values and directions, which shows the accuracy and reasonability of the geostress field regression. Thereby, the distribution of initial geostress field of dam region can be obtained, and then a rational 3D initial geostress field for excavating simulation and stability analysis of high slope and underground powerhouse of Jinping Hydropower Project is provided.