Abstract: The time-variant failure probability of the degraded bridge subjected to time-dependent resistance and load is investigated. A time-variant resistance model for the components of the existing concrete bridge in China is established considering reduction in area of corroding reinforcement, degradation of steel strength and concrete strength. A random load process is introduced. The adaptive importance sampling method is used to evaluate the time-dependent reliability index. Additionally, the effect of several parameters on the system failure probability is studied. The present study serves a reference to better predicting the service life of deteriorating RC bridges, and to developing optimal lifetime reliability-based maintenance strategies for bridges.