Based on time-dependent reliability theory, intermediate working state is considered, combining with current specifications, concrete filled steel tubular (CFST) arch bridge is discussed as a case, determination principle of three-stage mode is given. The direct correspondence from reliability indices to working states is made clear consequently. Introducing state variables, dynamic Monte-carlo method is presented. Basic formulas of failure probability and damage probability are derived by variable time increment method. The bridge is an CFST arch bridge with 83.6 meter-span, which operated for 10 years. According to test result, dynamic Monte-carlo method is used to estimate the reliability of this bridge. The analysis result shows that, in the tenth year, both damage reliability index and failure reliability index are above target reliability index. The example bridge is in safe working state at present. This conclusion is consistent with the facts.