Supercavitating vehicle utilizes supercavity to reduce the drag force when travelling underwater. It could achieve a speed higher than 200Kn. The dynamic equations of supercavitating vehicle in the vertical plane were studied and the forces are analyzed in detail in this study. Three possible balanced states and configurations are analyzed. Considering the deviation of mass center during the flight, trajectories of uncontrolled supercavitating flight with these configurations are calculated respectively. Results show that fin position and the deviation of center of mass are main factors that affect the stability of underwater trajectories. The distance between fins and center of mass should be as small as possible.