A real-time buoyancy measurement method is presented for a full-ocean-depth underwater robot to solve the difficulties in buoyancy calculation due to the change of density and the uncertainty of mechanical parameters at more than 10,000 meters depth. This method measures the buoyancy based on the propeller thrust and acceleration measurement. To minimize the measurement errors caused by the uncertainty of the dynamic parameters, the underwater robot need to perform a special motion. In this paper, the characteristics and application of the method are discussed, and an experiment is carried out at nearly 10,900 meters depth using the Hadal autonomous & remotely operated vehicle (Hadal ARV). The experimental results show that the average error of the measured result is 0.3 N, which satisfies the expected requirement.