Abstract: Natural circulation technology and helical-coiled tube steam generator are widely adopted in marine nuclear power system design due to the inherent safety feature of natural circulation and high heat transfer coefficient of helicalcoiled steam generator. The unique structural characteristics of helical-coiled pipe causes some special phenomena such as secondary flow effect inside the tube, and the heat transfer characteristics are more complicated than those in circular pipe. Under the environment of ocean motion, flow patterns and heat transfer characteristics of the fluid inside the helical tube will also change significantly under the influence of the nonlinear additional force. Thus relevant research is necessary to clarify the influence of ocean motion to the flow and heat transfer characteristics inside helical tube. Based on this background, a natural circulation system experiment loop under ocean motion was built, and the research on the natural circulation characteristics of the system and heat transfer characteristics of helical tube under ocean conditions was conducted in this paper. Firstly, experiment of single phase flow and heat transfer characteristics inside the helical tube under a variety of typical single and coupling ocean motions was conducted. The results show that driving force of natural circulation drops under inclination movement, and the steady flow rate and pressure drop are less than those of static condition, flow resistance coefficient under inclination condition is slightly lower than static condition. The influence of trim on the natural circulation capacity of the system is greater than that of heel. Average heat transfer characteristics inside the helical tube under ocean conditions were studied furtherly, it is found that the heat transfer capacity along the side near the central axis is significantly weaker than that along the side far from the central axis, which can be 24 times lower rolling motion, declines the average heat transfer coefficient of helical tube while pitching motion promotes it. An empirical relation of heat transfer coefficient in helical tube under ocean motion was established based on experimental data, and the error between the experimental value and predicted value is less than 10%. The influence of ocean motion on the occurrence point of natural circulation flow instability was studied, results show that inclination movement can make the occurrence of natural circulation flow instability ahead. Geometric model of the experimental loop was built employing a marine condition system analysis code based on RELAP5 code, and the transient simulation of rolling motion was carried out. Calculation results are in good agreement with the flow rate and pressure drop experiment data, which verifies the reliability of the program applied to natural circulation system analysis under ocean motion. Results of this study can offer reference for natural circulation analysis of floating and marine reactors and performance evaluation and design of helical-coiled tube steam generator under ocean condition, and also provide data support for the development of related analysis code.