Abstract: Excavation of the diversion tunnel No.2 of the Nuozhadu Hydropower Station passed through the faultage F3.During the process of excavation,the strain and stress distribution of adjoining rock were controlled by the mechanics parameters of rock mass around the influential cingulum of faultage F3.IN order to evaluate the stability of adjoining rock because of the excavation of layersⅡ and Ⅲ,the back analysis of mechanical parameters and the initial stress filed of the rock around the faultage F3 was needed.According to the back analysis of the tunnel excavation,the relationships between the monitored data of displacement in situ and the excavation of the tunnel should be considered,so the displacement increment of excavation was used in the back analysis,and the computing process of 3D nonlinear finite element method in which the average error of excavation displacement increment was used as the target function was analyzed.To decrease the error of the back analysis influenced by the displacement data in situ,the monitored displacement data were dealt by the multi-linear recursive model,and the precision of results of the back analysis were effectively enhanced.Finally,the mechanical parameters of the rock mass and the initial stress of filed the rock around the faultage F3 in diversion tunnel No.2 were analyzed.