Abstract: The natural circulation system is one of the important directions for the development of offshore power plants, and the natural circulation drive force is relatively small and is easily affected by ocean swing conditions. Therefore, it is meaningful to study the driving force and resistance characteristics of natural circulation under rolling conditions. Based on a typical natural circulation loop, the driving force and resistance model of natural circulation under rolling condition was established. The circulation loop includes hot sections and cold sections, and the fluid undergoes phase changes in the loop, so the model can reflect both single-phase flow and two flow processes. The driving force model included a single-phase driving model and a two-phase driving force model, considering the influence of rolling. The resistance model included the commonly used single-phase resistance model and the L-M two-phase model. The model matches well with the experimental data, The pressure drop error between the experimental value and predicted value using the model of the single-phase section does not exceed ±10%, and the pressure drop error of most conditions of the two-phase section does not exceed ±20%. It is found using model that the natural circulation driving force and system flow fluctuate periodically under rolling condition, and the fluctuation amplitude of driving force and rate of flow increases with the increase of rolling amplitude and the shortening of rolling period. When the swing angle is the most negative, the driving force and rate of flow are close to the peak value. The total driving force under the rolling condition is the sum of gravity driving force, centripetal driving force and tangential driving force. Within the working conditions, the centripetal driving force is negligible, the total driving force is influenced by the coupling effect of gravity driving force and tangential force, and the smaller the rolling period, the greater the impact of tangential force. The total pressure drop and partial pressure drop of two-phase flow in natural circulation fluctuate periodically, and the changes of the position of the flow channel and the average density of the two-phase flow cause periodic fluctuations in gravity pressure drop, acceleration pressure drop, and friction pressure drop. When the rolling angle is most negative, the gravity pressure drop and friction pressure drop are close to the minimum value, while the acceleration pressure drop is close to the maximum value. The rolling period has much influence on the fluctuation period, but little influence on the fluctuation amplitude of the pressure drop.