Compared with the conventional solid rocket motor which works in atmosphere, the underwater solid rocket motor has different performance characteristics, because the buoyancy of water cannot be neglected, while in the atmosphere only gravity is present. In order to get a deep understanding of the mechanism of thrust oscillation characteristics and of the jet structure of underwater solid rocket motor, a 3-D numerical simulation using the volume of fluid (VOF) model is given in this paper to investigate the horizontal jet structure and the oscillating flow field induced by the gravity and buoyance. The principle of momentum is carried out to analyze the oscillating flow field in the initial working period. Result of analysis indicates that the behavior of rising gas bubbles is remarkably influenced by gravity and buoyancy. Compared to the conventional solid rocket motor in the atmosphere environment, the phenomenon shows that the rising gas bubbles are more obvious in the water. It is concluded that thrust-gravity and buoyancy coupling can generate bigger roll moment in the vertical direction than the conventional solid rocket motor. Numerical calculations in conjunction with experiment in the references proves that the model contains gravity, buoyancy and the VOF model can be used to describe the jet structure and thrust oscillation of underwater solid rocket motor; gravity and buoyancy cannot be ignored in the research of underwater solid rocket motor performance characteristics.