Abstract: An ocean-based thermal-hydraulic analysis code was developed based on RELAP5/MOD3.1 through the establishment of additional force model of ocean conditions and control volume coordinate solving model. The natural circulation characteristics of integrated PWR-200 (IP200) under ocean conditions were studied. The effects of inclination and rolling motions were analyzed. The results conclude that the inclination condition can reduce the core flow rate and lead to inconsistent coolant flow rates of the left and right loops. Furthermore, it affects the heat transfer of once-through steam generators. In the case of rolling motion, the additional pressure drop of the loop is dominated by tangential force. The off-center roll axis location and the combination of the inclination and rolling motions can both break the thermal-hydraulic symmetry among different loops and enlarge fluctuation amplitude of the core flow rate.