The buoyancy of deep-sea AUV (autonomous underwater vehicle) changes significantly because of the increasing diving depth caused variation of temperature, salinity and pressure. The buoyancy change will greatly affects the sailing performance of the AUV. Aiming at this issue, the main factors influencing the buoyancy are analyzed independently, and the relationship between each factor and the buoyancy change is given. Then influences of each factor are integrated, and the quantitative relation between the buoyancy changes and the diving depth in diving process of 6000m depth rating AUV Qianlong I are calculated according to the sea trial data of Qianlong I. The calculation results indicate that: the pressure has the primary impact on the buoyancy change, the buoyancy increasing linearly with pressure; the influence of the temperature on the buoyancy change (related to the sea surface temperature) is secondary, and the relationship is nonlinear; the salinity has the least impact on the buoyancy change, and its influence on the buoyancy change decreases with the increase of diving depth. The correctness of the analysis results are proved by the sea trial, and the efficiency of the AUV balancing can be effectively improved.