Abstract The press in ocean deeper than 10,000 meters brings a great challenge to trim and buoyancy measurement for underwater robot. The Hadal Autonomous & Remotely Operated Vehicle (Hadal ARV) firstly measured its remaining buoyancy under nearlly 10,900 meters depth in ocean as the first time in China. Unknown temperature and salinity can cause inaccurate in density computing. The application of new technologies and new materials makes buoyancy estimation more difficult. Space of full-ocean-depth underwater robot is limited, the sensor type and performance are subject to a lot of constraints. The method present in this paper is a real-time buoyancy measurement method for a full-ocean-depth underwater robot, which is based on propellers and an accelerometer for buoyancy measurement. Through the implementation of special motion movements, to avoid the the uncertainty of dynamic parameters of the measurement results. In this paper, the characteristics and application of the method are discussed, and the experimental results show that the average error of the measured result is 0.3N, which can evaluate its buoyancy situation appropriate.