Abstract: According to the thermal-hydraulic analysis of sub-channel, additional force models of heaving, rolling and complex movement under ocean conditions were established and applied to the sub-channel momentum equation in COBRA ⅢC sub-channel analysis code. The inlet boundary of ocean conditions was also established in this paper. The critical heat flux ratio, exit void fraction and coolant flow of the hot channel were calculated for MUTSU reactor and experiment heating tube, and the results were compared with the experiment data from literature. Calculated thermal-hydraulic characteristics of MUTSU reactor were studied under ocean conditions. The research results show that thermal-hydraulic parameters are varied periodically under fluctuating of ocean conditions. The heaving motion has a large impact on pressure drop of the sub-channel. The sub-channel coolant flow and temperature are greatly influenced by swing motion.