In order to effectively evaluate existed bridges' reliability, the effect of bridge service time on bridge reliability was considered. To extend the so-called semi-stochastic process model for specified in the design code in analysis reliability for existing structure, assuming that the structural resistances and the action effects are independent, based on the characteristics of the resistance and action effects variation with time, considering the resistance is an independent increment process and calculating the self-revelation coefficient of resistance, an approximate algorithm of failure probability was developed. And a fully stochastic process model was developed to analysis time-dependent reliability. Through the example,this method is simple and convenient and can supply a basis for structural assessment and service life prediction.