In this paper, a method of vertical dynamic parameters identification based on flexible load is proposed for the deepsea application. The vertical dynamic parameters of the robot can be calculated at a depth of 11,000 meters. In this paper, the vertical dynamical identification experiment of HADAL ARV is introduced on the seabed near 11000 meters. Finally, the real-time residual buoyancy of the robot is calculated according to the obtained dynamic parameters, which verifies the effectiveness of the method.