The in-situ stress is one of the most important factors for the design and construction of large scale underground structure group. Based on the engineering geologic characteristics and the measured data of in-situ stress in the region of Dagangshan Hydropower Station

the lateral earth pressure coefficient model for 3-D in-situ stress inversion is established taking into account the influencing factor of in-situ stress. A methodology of inversion analysis combining the Nelder-Mead algorithm and finite element method is proposed

in which the finite element program is embedded as a module in the Nelder-Mead algorithm. Then

a new exact penalty error model is established between the actually measures and corresponding calculated results of stresses. Through the finite element approximation analysis

the optimum inversion coefficient can be derived. Through the comparison between calculated and measured stresses of measuring points

it is shown that the calculated stresses are similar with the measured stresses

which suggests that the reasonability of in-situ stress field finite element approximation. Therefore

the calculated stress field can be used for the analysis of the excavation simulation and long-term stability of Dagangshan Hydropower Station.