The reliability of the existing tunnel will decline gradually during service period due to various factors. This paper analyzes the structural reliability characteristics of the existing tunnel, the time-varying performance function of the reliability of the existing tunnel is proposed. The statistical value of lining action effect is calculated by using Monte Carlo Method combined with the finite element method .By analyzing the factors which influence the resistance degradation of tunnel lining, at the same time consider the time variability of the concrete strength deterioration and random changes of cross section size, the statistical parameters of each variable during follow-up service period is calculated by using Bayesian Method. The ultimate limit states equation of the concrete lining is established. The reliability index of the lining structures of an existing is calculated and the results show that the reliability decreases with the service time getting long.