Abstract: When a full-ocean depth ARV (autonomous and remotely-operated vehicle) dives to Mariana Trench, the vehicle structure and the volume of compensation oil will change due to the pressure increase and temperature fall of seawater in the working environment. Taking the abyssal seawater density, gravity acceleration and robot volume change into consideration, the change amount of the robot buoyancy state is calculated, so as to calculate the robot buoyancy trim. A calculation method of buoyancy trim for the full-ocean depth ARV is obtained through formula derivation, combining with the experimental data of Hadal ARV and the compression experiment data of compensation oil. It mainly includes seawater density calculation, Mariana trench gravity acceleration calculation, the volume change measurement of the robot and compensation oil. The ballast for buoyancy trim at 11 000 m depth is calculated to be 8.9 kg. The buoyancy trim of Hadal ARV ensures that the buoyancy and gravity match suitably in abyssal working environment. Several sea trials at Mariana Trench are conducted. The proposed method provides a theoretical calculation basis for buoyancy trim of the full-ocean depth underwater robots.