Abstract: The load distribution and the water-existing attitude of the vehicle will be affected by the natural cavitation on the shoulder of vehicles during the water-exit process. In engineering, active ventilation by opening vent at shoulder is often employed to improve the mechanical environment of vehicle surface so as to solve such problems. This article is aimed at solving the problem that the ventilated cavity circumferential coalescence of underwater vehicles is unsatisfactory. Using the VOF (volume of fluid) multi-phase flow model and dynamic grid technique of dynamic layering based on finite volume method, the mechanism of adding a small-scale annular groove at the downstream of the vent to promote the cavity coalescence was numerically investigated. And the promotion effect of the annular groove on cavity coalescence at different development stages and different working conditions was also studied. The results show that the cavity coalescence is greatly improved by the annular groove. Flow separation in boundary layer occurs when the incoming flow passes through the annular groove in the moving coordinate system because of flow expansion. The induced entrainment of annular groove, on the one hand, retards the axial development of cavity, and promotes the generation of a larger circumferential shear vortex along with expanding in the circumferential direction. On the other hand, part of the ventilation gas is sucked into the annular groove. The gas sucked into the groove is squeezed and broken contributing to the circumferential coalescence. The coalescence cavity in the groove overflows and leaks to promote the upward movement of the cavity coalescence boundary. In addition, under different working conditions, the shape and internal pressure of the cavity become more stable for the groove improving the internal flow state of the cavity.