In order to determine the installation opportunity of the secondary lining for the exit section of the left tube of Fuxi tunnel when the deformation of the rock mass is still unstable, the internal forces and safety factors of the lining structure are calculated on basis of the residual deformations of the tunnel that are estimated from the measured results of the tunnel deformation. The results indicate that the magnitude of the deformation occurring after the installation of the lining has great impact on the safety and internal forces of the lining structure. If the deformation of the lining is only 6.5% of the residual deformation, the stability of the secondary lining can be guaranteed. It is found that the internal stress of the lining obtained by means of field monitoring is smaller than the strength of the concrete lining. It is concluded that the secondary lining can be installed when the deformation of the rock mass is still unstable, provided that the construction quality of the lining can be guaranteed.