Operating in the depth-varying oceanic environment, the buoyancy of deep-sea underwater gliders (UGs) will change with depth due to pressure hull deformation and seawater density variation. As the buoyancy variation caused by these two factors is of the same order of magnitude as the nominal net buoyancy, hull deformation and seawater density variation will accordingly affect the dynamic behaviors of deep-sea UGs. In this paper, a full dynamic model was established using Newton-Euler method for a deep-sea UG, PETREL-II. The hull deformation and seawater density variation, fitted as the functions of depth, are considered in the model. Comparisons of results obtained by sea trials, the full dynamic model and the simpler dynamic model without considering hull deformation and seawater density variation showed that the proposed full dynamic model can more truly reflect dynamic behaviors of the glider. Comparisons of simulation results for the full and simpler dynamic models showed hull deformation and seawater density variation have great effect on pitch angle and velocity of the gliders. Through analysis of motion control strategy, a buoyancy compensation scheme was proposed to reduce the negative effect of hull deformation and seawater density variation, and was validated to be effective by sea trials.