Abstract: When a superspeed underwater vehicle runs in cruise phase, its motion mode is different from that of a usual bedewed vehicle because a majority of its surface is surrounded by supercavitation. Therefore, its motion characteristics are unique. In order to control the depth and the attitude of this kind of superspeed underwater vehicle, it is necessary to investigate the motion characteristics of the vehicle on a vertical plane. This paper analyzed the hydrodynamic characteristics of the vehicle based on the hydrodynamic mechanism, and established a motion model of the vehicle on a vertical plane. It also analyzed the motion characteristics with lack of damp, and presented a key problem that the attack angle is not allowed to exceed the bound in control process. It was suggested that the control force should not be too violent. The research may be useful for designing the control system of a superspeed underwater vehicle.