Recently, nuclear-powered ships have attracted attention due to international regulations on greenhouse gas emissions and the trend of rapid and large-scale ships. In order for a nuclear system to be used in marine propulsion, it is important to achieve small in size and should be able to respond to rapid load demand changes. In this study, a super-critical carbon dioxide (S-CO₂) cycle is proposed as a power conversion system for pressurized water reactors (PWR) for marine propulsion. The S-CO₂ cycle has been attracting attention as the next-generation power conversion system that can be an alternative to the steam Rankine cycle due to its small system size and higher efficiency. The conceptual design of the S-CO₂ power conversion system is first performed under the reference reactor conditions, including cycle design and component design. To conduct system analysis, MARS-KS code, one of the nuclear thermal-hydraulic safety analysis codes developed by Korea Atomic Energy Research Institute (KAERI) and actively used for the safety review calculation of a nuclear power plant by Korea Institute of Nuclear Safety, is improved to accurately simulate the S-CO₂ power conversion system combined with PWR. The PID controller based automatic control strategy of the entire system is designed to respond to rapid load changes. Transient analyses are performed with the newly developed system analysis code under various scenarios.