Current bridge evaluation methods can give the bridge load-carrying capacity at the moment of field inspection and load testing. However, the evolution rule of the load-carrying capacity is also important since these testing cannot be implemented frequently. In order to accurately predict the bridge load-carrying capacity after load testing, this study first introduces the bridge initial load-carrying capacity calibration method by diagnostic load testing based on Chinese and US evaluation code. Then, the resistance degradation model is established to simulate the degradation path of bridge resistance based on the Gamma process. Combining the results of the initial resistance calibration and the degradation model, the time-varying bridge safety factor prediction method is proposed. Finally, the proposed method is verified through a load-carrying capacity prediction example of prestressed concrete continuous box girder bridge, and the results of the safety factor prediction in the unfavorable cross sections is given combined with diagnostic load test data. The outcome of this paper can provide a basis for management decisions such as bridge reinforcement, demolition, and replacement after bridge load testing.