Abstract: The natural circulation theoretical models of nuclear machinery under ocean condition were established, which mainly include two-phase, non-homogeneous, drift flux model with five equations. The additional pressure drop induced by ocean condition was illustrated by modifying the mixture momentum equation, the nodal pressure solution was used to solve the pressure matrix, the non-isothermal model was preferred to calculate the two-phase friction factor, and the influence of shaking motion to which was also taken into account. The local loss coefficient was calculated as a function of Reynolds number. The influence of shaking and heaving motion to heat transfer coefficient was also considered. The two-group, 3-D neutron kinetics model was established. A nonlinear iterative semi-analytical nodal method was used to solve diffusion equation. The natural circulation operational characteristics of nuclear machinery under rolling, pitching, heeling, heaving, linear acceleration and rotation motions were investigated.