The component and mission of CODOG propulsion system were analyzed in this paper. According to the mission variation, different operation conditions were introduced and mission reliability models of each condition were established by the use of reliability theory. After reliability data of main component of CODOG propulsion system been obtained, mission reliability of each condition was calculated and its phased mission was simulated through MATLAB software. Relative curve between system reliability and working time was found. The results show that, mission reliability of cruise condition is the biggest while phased mission reliability is the smallest. System availability is bigger than reliability after taking maintainability into account. System reliability can increase to a great extent and its useful life is longer by improving maintainability of its component.