In response to the problems of boiler low water level and pump cavitation caused by the leakage of feedwater unloading pipeline in the process of boiler load raising of the conventional steam power system, the relational model of water-lift, mass flow, revs of booster pump, and each impeller of the feedwater pump are identified by using a differential evolution algorithm based on residual correction. The relational model of mass flow and resistance of feedwater pipelines and valves, the calculational model of working fluid parameters in feedwater system, and the performance degradation model of feedwater unloading control valve are established by using be mechanism modeling method. On this basis, the process of boiler load raising is simulated, and the main parameters of feedwater system are obtained at different feedwater unloading flow rates. It is found that the performance degradation of the control valve of the feedwater unloading pipeline is one of the main causes for boiler water low level and pump cavitation in the process of boiler load raising. In order to further clarify the degradation rule of the performance reliability of the feedwater system with the working time, a simulation method integrating the mathematical model and Monte Carlo random sampling is adopted to the performance reliability simulation of a certain type of marine water supply system in the process of boiler load raising. Thus the whole performance reliability changes of this boiler and the performance reliability life of this feedwater unloading control valve are obtained. The research results reveal the mechanism and degeneration rule of a unique fault of the steam power system, which has certain theoretical and engineering application value.