Abstract: The unknown but bounded property of the measured data and mechanical parameters is considered and interval variables are used to denote the measured data and mechanical parameters in this paper. The interval parameter perturbation method is presented to solve the equations of interval FEM and the interval back analysis model is put forward. Based on the interval parameter perturbation method, the mean and deviation of the mechanical parameters are obtained by the two-step back analysis. For the example of the mechanical parameter back analysis of surrounding rocks of the model cave in the large hydroelectric power station, the effect on the measure precision is studied. It can be found from the result that the method is reasonable and feasible.