Abstract: Initial geostress field is very important for engineering design and stability analysis of underground engineering. The back analysis method is often adopted to get the whole geostress field of engineering zone based on the limited measured data. Based on the measured geostress data, geographic and geomorphic conditions and characteristics of geologic framework, the distributing characters of geostress field in underground powerhouse zone of Dagangshan hydropower station are studied. Furthermore, the multiple regression method is applied to back analyze the three dimensional geostress field of underground caverns of Dagangshan hydropower station; and the distributing rule and influence factor of geostress are also discussed. The results show that: the geostress field in underground powerhouse zone of Dagangshan hydropower station is affected by tectonic stress, geologic structure, terrain and physiognomy; among them, lithology has little influence; dyke and fault have great influence; the value of geostress is medium and there is , where is maximum horizontal principal stress; is minimum horizontal principal stress; is vertical stress; the lateral pressure coefficient vertical to powerhouse axis is 0.5～0.6, along powerhouse axis is 1.1～1.3

Key words: geostress, multiple regression method, back analysis, Dagangshan hydropower station