Abstract: Existing bridges undergo time‑varying load effects and resistance degradation during service. The complex loads and diverse failure modes make the existing bridges face greater risks in service. Therefore， it is urgent to make time‑dependent reliability assessment for the service bridges. The classical time‑varying reliability analysis method is more complex and difficult as the number of random variables increases. In this paper， probability density evolution theory is introduced to solve the above problem， which is more advantageous for solving the reliability of complex structures with multiple random variables. The dynamic reliability of the existing bridge in serviceability limit state and ultimate limit state is analyzed by considering the bridge resistance degradation and load effect increase， as well as the time‑varying factors such as shrinkage and creep effect of concrete bridges. The accuracy and computational efficiency for this method are compared with the Monte Carlo method， and the effectiveness of the proposed method is verified.