The invention relates to a wet deck slamming forecasting method applied to a side wall type hovercraft. The method comprises the steps of: (1) respectively generating an incident wave, a ship motion and an air cushion pressure which are acquired from an outside world into Fourier series according to the time sequence; (2) calculating a making wave of an inner free surface of the side wall type hovercraft for a harmonic wave, which is acquired in step (1), corresponding to each series; (3) linearly superposing making wave components acquired in step (2), and subtracting a vertical component of the ship motion and a wet deck height to obtain the time sequence of a displacement of the inner free surface relative to a wet deck; and (4) according to the displacement of the inner free surface relative to the wet deck, counting slamming times and slamming positions of the wet deck, calculating corresponding slamming loads and obtaining a wet deck slamming forecasting result for the side wall type hovercraft. According to the wet deck slamming forecasting method applied to the side wall type hovercraft, a three-dimensional hovercraft model is converted into a non-constant slicing model in a two-dimensional time domain, the dimensionality is reduced and the calculation efficiency of the making wave of the inner free surface and even slamming of the wet deck is improved.