Abstract: A ship is affected by the transient ocean motion during navigation, and the two-phase flow is often in a transient motion state. The two-phase flow pattern may be different from the land-based conditions, which may affect the flow and heat transfer characteristics. Therefore, it is significant to accurately distinguish the two-phase flow pattern under transient motion conditions for the calculation of two-phase resistance and heat transfer. In this paper, by using a separated flow model, the momentum equations for the liquid film and gas core of annular flow were listed separately. Meanwhile, based on introducing the transient external force field generated by the motion under two typical ocean motion conditions of rolling and heaving and considering the effects of buoyancy and additional inertial forces, a transition criterion which was suitable for churn flow to annular flow under transient motion conditions was also proposed. This criterion can also be applied to vertical upward and inclined flows. The calculation results of the transition criterion model were verified through experimental data under static and transient motion conditions, and there is good agreement with the experimental results. The results show that when the superficial liquid velocity is less than 0.35 m/s, the transition curve approximates a vertical curve, while the superficial liquid velocity is larger than 0.35 m/s, the thickness of the liquid film increases, and the axial motion of the liquid phase plays a major role in the flow pattern transformation. Similarly, by comparing the calculation results of the transition from churn flow to annular flow under different motion conditions with experimental data, it can be found that within the existing parameter range, the additional force introduced by rolling motion has little effect on the continuous gas phase at the center of the channel. Therefore, although the thickness distribution of the liquid film on the wall is uneven due to the influence of rolling motion, the annular flow structure will not be damaged, and the transition curve between churn flow and annular flow is basically consistent with the vertical upward direction. Moreover, with the increase of the heaving amplitude and the decrease of the heaving period, the transition boundary curve of the churn flow towards the annular flow shifts to the right. This is because the additional acceleration introduced by the heaving increases, and the liquid film is subjected to instantaneous volume force in the vertical direction and oscillates periodically, making it easier to fall. Therefore, a larger gas phase apparent velocity is required to maintain a stable annular flow, so the transition boundary shifts to the right. By analyzing the transition criteria, a churn flow to annular flow transition curve is given, and the influence of motion parameters on the transition boundary is obtained.