Abstract: Moving on the sea, the ship can be heavily affected by sea waves, which results in rolling, pitching and other different motions. Meanwhile, the randomness of sea waves can also cause the change of ship's motion parameters, such as the period and amplitude of pitching or rolling, which can disturb the ship flow field and increase the risk of the ship-borne helicopter's operation above the flight deck. In order to ensure the safety of the take-off/landing and other operations on the deck for ship-borne helicopters, the flow field over the flight deck of a simplified amphibious assault ship in the pitching motion was numerically simulated using computational fluid dynamics method. The irregular pitching motion was simplified as a periodic harmonic motion, and the dynamic mesh method was employed to simulate ship's rolling motion. The development of vortex structures over the deck and the influence of the change of the pitching period and amplitude on the ship airwake were analyzed qualitatively first. Then the dynamic ship airwake was further analyzed quantitatively by the change of vertical velocity. When the ship suddenly begins to pitch or its pitching period is halved, the establishment of the periodic flow field near the superstructure and stern will lag behind that near the bow. At the same time, the acceleration of pitching angular velocity leads to the deterioration of the flow field. When the pitching amplitude is halved, the ship is in the "rest period". The flow field changes gently and the variation of vertical velocity is significantly reduced at this state, which is conducive to the landing of the ship-borne helicopter. For different pitching states, the standard deviations of vertical velocity at the observation points have an increasing trend as the height decreases. It is worth noting that the fluctuation of vertical velocity at the bow near the deck is the strongest. The pilots of ship-borne helicopters should pay attention to the influence of the sudden change of pitching motion, and take corresponding measures to ensure the safety of deck operation.