Abstract: The safety of helicopter's take-off and landing on ship has been widely concerned. The coupling of ship airwake and rotor vortex has a great threat to the operations of shipborne helicopter. In order to overcome the limitation of steady simulation and momentum source method, the overset mesh method was used to numerically solve the unsteady flow field where helicopter is hovering or landing on a ship. The results show that the overset mesh method can capture the details of rotor vortex more accurately. During the hovering, the rotor thrust coefficient gradually converges and appears as stable periodic fluctuations. The tip vortex falls off spirally and interferes with the recirculation zone behind hangar as well as the ship side vortex, which shows strong unsteady effect of the flow field. During the landing, the reattachment point of recirculation zone first moves towards the stern and then moves towards the bow. The rotor thrust coefficient violently oscillates due to the collision between tip vortex and ship vortex intensifies. The turbulence intensity of the coupled flow field increases sharply and the oscillation amplitude becomes larger and larger, which adds risk to land on the ship.