Abstract: Affected by sea wind and waves, the floating nuclear power plant (FNPP) would be in rolling motion during operation. The rolling motion will introduce fluctuated inertial force periodically, resulting in periodic thermal and hydraulic parameters variation, and may threaten the safety of the reactor. To investigate the influence of rolling motion on the heat transfer characteristics in the rod bundle channel under natural circulation condition, an experimental system with a 5×5 rod bundle channel was design and built on a rolling platform. An experimental investigation was conducted to study the influence of rolling motion on heat transfer in 5×5 rod bundle channels under rolling motion. The transient heat transfer characteristics of natural circulation in rod bundle were obtained. Experimental results demonstrate that the rolling motion induces periodic oscillations in the flow rate of the system. Furthermore, it is observed that with an increase in heating power, there is a reduction in the relative amplitude of these flow rate fluctuations. Consequently, this leads to periodic variations in Nu, accompanied by a corresponding decrease in relative fluctuation amplitude as well. The fluctuant flow disturbances disrupt the boundary layer and yield an enhancement of approximately 5% in the heat transfer coefficient. Regarding the local heat transfer characteristics, the incorporation of rolling motion introduces secondary flow which can significantly alter them. The presence of gravity and centrifugal force field fluctuations with a period half that of the rolling period leads to a local Nu curve with two peaks. With the center of the rod bundle channel as the axis, reverse phase fluctuations occur on both sides of the channel. The heat transfer performance is enhanced in subchannels near the rolling axis due to the rolling motion, but it diminishes away from that axis. The secondary flow induced by rolling motion amplifies velocity fluctuation amplitude in subchannels close to the rolling axis. Local Nu increases with an increase in power, whereas relative fluctuation amplitude decreases as power rises. The troughs of Nu are almost the same under different rolling periods, and the peak Nu fluctuates observed in subchannels become more pronounced as the intensity of rolling motion intensifies. In conclusion, this paper’s experimental results significantly contribute to advancing our understanding of heat transfer in rod bundle channels under oceanic conditions. The experimental results of this paper can provide experimental data for the study of wall temperature distribution of the rod bundle channel under rolling motion conditions.