Abstract: The initial geostress is the main source of force influencing the deformation and stability of underground powerhouses and also one of key factors to be considered in engineering design and construction, especially in construction of underground powerhouses of large hydropower projects. Based on the geological conditions and measured geostress data of an underground powerhouse of a certain hydropower project, the finite element method is applied to analyze the initial geostress of the underground powerhouse. It is shown that the action of the horizontal geological structure is preferably dominant in the region of the underground powerhouse, and the geostress is composed of the gravitational stress and the tectonic stress. Through comparison between the calculated and measured geostresses of measuring points, it is found that these two are similar in values and directions, which suggests that the geostress field obtained by regression is reasonable and conforms to the historical background of geological structure. The results of the geostress field can be applied to practical engineering projects.