In order to prevent the vertical launched vehicles under deep water departing from trajectory in theory, the stability of trajectory is studied. Based on the stability theory and underwater trajectory model, the stability is simulated and analyzed in different conditions. Due to the buoyancy-term is important to the stability of uncontrolled vertical launched vehicles, the influence on stability of trajectory of the center of buoyancy is studied, and impact of determination of the relation among the three center of mass, pressure and buoyancy upon stability of trajectory can be clarified, and the center of buoyancy must be in front of the mass center and the determination of them must be long enough for vehicle with relaxed static stability. The study may contribute to the engineering design of hydrodynamic structural layout and the stability studies of trajectory in theory.