The problem focused on depth control for ROV is studied, and the adaptive terminal sliding mode controller based on nonlinear disturbance observer(NDO) is proposed. The design process is described in detail. By using the Lyapunov stability criterion, global asymptotic stability and tracking error convergence are proved when external disturbance and model uncertainties exist. Simulation studies show that the proposed controller can overcome and estimate factors such as external disturbance and model uncertainties well, and has good robust performance and the ability of rapid convergence for depth changing at any time.