Abstract: 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 using the analysis results of each factor and combined the data of the sea trial of the 6000m depth rating AUV Qianlong I, the buoyancy changes with variance diving depths are calculated quantificationally in diving process for Qianlong I. The calculation results indicated 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 had the least impact on the buoyancy change, and the influences on the buoyancy change reduced 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.