A wing-in-ground (WIG) craft is designed to fly at low altitudes, using the ground effects to enhance its aerodynamic performance, thereby reducing fuel consumption and improving overall efficiency. However, incorporating a stepped hull into a WIG craft can increase aerodynamic drag during flight. The steep geometry of the stepped hull causes flow separation, reducing the craft's aerodynamic performance. This study aims to evaluate the impact of using a cavity as the passive flow control mechanism on the aerodynamic performance of a WIG craft with a stepped hull. Four numerical simulations are conducted to investigate the effects of different cavity configurations. The findings indicate that cavities are able to enhance the aerodynamics of WIG craft, particularly behind the backward-facing step of a stepped hull. A cavity with an aspect ratio of 2, positioned at a distance of 2h from the step, can reduce the drag coefficient by up to 5.11% and increase the lift-to-drag ratio, C_L/C_D by up to 5.17% compared to the conventional WIG craft.