A four-degree-of-freedom mathematical model was established to investigate the course stability of an air cushion vehicle (ACV). The forces of aerodynamic,propeller and rudder were obtained by wind tunnel experiments. A series of constrained model experiments with horizontal-planar-motion-mechanism (HPMM) were conducted to determine the hydrodynamics. The ACV’s course stability was analyzed by using Hurwitz deter-mination method with differential equation of perturbation,and the four-degree-of-freedom course stability was then checked by simulated calculation. The analysis and simulation results show that the course stability of fourdegree-of-freedom is applicable to ACV and is stricter than that of two-degree-of-freedom. A four-degree-of-freedom mathematical model was established to investigate the course stability of an air cushion vehicle (ACV). The forces of aerodynamic, propeller and rudder were obtained by wind tunnel experiments. A series of constrained model experiments with horizontal- The ACV’s course stability was analyzed by using Hurwitz deter-mination method with differential equation of perturbation, and the four-degree-of-freedom-course stability was then checked by the experiment The analysis and simulation results show that the course stability of four degree-of-freedom is applicable to ACV and is stricter than that of two-degree-of-freedom.