In nuclear power systems, the natural circulation loop startup to remove the residual heat after losing the power for the forced circulation; however, under the rolling condition, the system will encounter additional forces, including the transport force and the Coriolis force, which may change the driving force and break the stability of natural circulation. In this work, the CFD model for a natural circulation loop was developed to study the dynamic startup performances and the pseudo steady-state operations under rolling conditions. Using CFD method can avoid employing the correlations for wall friction and heat transfer using in system code, which may be not applicable for rolling systems. Based on the CFD analysis, parametric studies were carried out, including rolling period, maximum rolling angle, heat power, rolling center and the rolling phase difference. The rolling angle, rolling period, rolling phase difference and heated power can significantly affect the thermohydraulics and stability of the natural circulation systems, while the rolling center has little influence. This work proved that CFD methodology may be qualified for analyze the natural circulation system under rolling conditions.