For the defects that the outlet pressure of condensate pump fails to meet the prescribed requirement during startup period of a marine feed water unit due to the fact that high temperature deoxidizing water enters the condensate pump and its suction pipeline during the standby state and the standby switching start-up, which leads to the cavitation of condensate pump, and on the basis of the head-volume flow rate-rotation speed model of each impeller obtained by Differential Evolution algorithm, the model of condensate system pipeline and valve, and the whole condensate system simulation model were established with SimuWorks simulation platform, and the changes of parameters such as enthalpy, pressure, temperature and the rate of vapor content with the switching process of the unit were obtained by simulation under the condition of different high temperature deoxidizing water leakage. The results show that the startup condensate pump will cavitate and its outlet pressure cannot be established in time when the condensate pump is filled with hot water before startup due to the leakage of suction gate valve, as well as the leakage of high temperature water from the startup unit and the outage unit which has lost drainage ability is large. Thermodynamics is used to reveal a special fault mechanism of marine condensate system, which has important reference value for system design improvement and fault detection.