Abstract: The compact design of the marine nuclear power reactor results in a significant reduction in the size of the main equipment, particularly the steam generator. The primary steam-water separator is the core component in this regard. The swirl vane separator is the primary separator in the steam-water separator, responsible for more than 80% of the steam-water separation task. The performance of the marine nuclear power system directly affects its safety and economy. The primary focus of this study centered on the swirl vane separator, employing the Euler-Euler two-phase flow model in conjunction with the RNG k-ε turbulent flow model. Furthermore, an additional inertia force resulting from the rolling motion was integrated into the momentum equation as a source term using user-defined functions (UDF). The impact of varying gravity components was considered, leading to the development of a three-dimensional numerical computational model to analyze the flow and separation of steam and water within the separator. The impact of varying rolling angle and rolling period on the flow properties and operational efficiency of the separator was methodically investigated. The results indicate that the turbulent mixing of the steam-water two-phase fluid within the separator is enhanced by the rolling motion, leading to a chaotic pressure field, velocity field, and distribution of liquid volume fraction. The critical state is observed when the rolling angle reaches 40° and the rolling period is 2 s. This critical state causes fluid backflow in different parts of the separator, greatly affecting the effectiveness of separating the two-phase flow. The additional inertia force from the rolling motion results in periodic pressure and velocity fluctuations within the separator, with a phase delay of one-quarter period compared to the separator’s motion. In the near-wall region of the separation cylinder, the liquid volume fraction reaches its peak when the separator is at its maximum inclination angle. The periodic changes in separation efficiency follow a sinusoidal pattern. Before reaching the critical condition, the variation in separation efficiency corresponds with the rolling motion of the separator. The influence on the separation performance becomes more pronounced with higher rolling magnitude and shorter rolling period.