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Based on in-situ observed deformation data in underground powerhouse constructions and the response surface approach, back analysis procedure for estimating geotechnical parameters of rock mass is proposed. The relationship between unknown geotechnical parameters of rock mass and deformations of some observing point is approached by the response surface procedure. The excavation process is divided into 13 operation steps for simulation of the stress redistribution due to excavation. The variations of the stress sate and deformation field in the rock mass due to the excavation is simulated through a finite element analysis. By defining objective function for back analysis procedure to make the difference between computed deformations and observed ones minimize, the inverse problem of parameter identification is changed into optimization problem. By using optimization algorithm, the stress ratio and elastic modulus are determined. The practical application results show that the forecasted deformations by finite element method agree well with observed ones in-situ. The effectiveness of proposed back analysis procedure for estimating geotechnical parameters of rock mass is validated.