Under the deep ocean environment the buoyancy of autonomous underwater vehicle （AUV）changes with the temperature，salinity and depth，which could affect its navigation performance seriously，in order to adjust the impact on the AUV，buoyancy adjustment device was developed to conquer this problem. In this paper，we put the AUV based on the variable buoyancy system as the research object. First based on data of the experiment in south China sea，the following three function relationships，i.e.，the relationship between the buoyancy deviation and depth，the relationship between the buoyancy deviation and temperature，and the relationship between buoyancy deviation and salinity are obtained. Second，after putting the buoyancy variation caused by depth into AUV mathematical model and simplifying the model we get the AUV hydrodynamic equations. Finally，the PID control，fuzzy control and fuzzy adaptive PID control were designed to study the depth control of AUV. Considering the rise time，accommodation time，overshoot and anti-interference，comparison and analysis about the above three methods are conducted. It turns out that the fuzzy adaptive PID control takes an overwhelming advantage over the PID control. Key words：autonomous underwater vehicles（AUV）；variable buoyancy system；variable
