[Objectives] Stern flaps have significant impact on the water performance of the amphibious platform. The influence of stern flaps on the motion and hydrodynamic characteristics of high-speed amphibious platform in the sliding state was discussed. [Method] Using the SST k-ω turbulence model and overlapping grid technology, the CFD numerical simulations of amphibious platform’s high-speed navigation under static water were performed, the motion response and stability of amphibious platform with different stern flaps were analyzed. Considering the influence of velocity, center of gravity and angel of stern flaps on the longitudinal motion of the platform, support vector machines were adopted to classify and recognize the boundary of its motion stability. [Results] The results showed: the stern flaps reduced the sailing trim angle by changing the pressure distribution on the underside of the platform, and the larger the rotation angle of the stern flaps, the more significant the influence on the motion stability; when the position of gravity center is the same, the maximum speed of the platform in stable state is increased. [Conclusions] The applications of tail appendages and the improvement of motion stability are of great significance to the high-speed development of amphibious platforms.