A nonlinear disturbance observer based backstepping sliding mode depth control algorithm is proposed for hovering operation of unmanned underwater vehicles（UUVs）in shallow marine environment. Firstly， based on the hovering characteristics of a fully actuated UUV，a five-degree-of-freedom decoupled heave channel dynamic model is established. Then，a nonlinear disturbance observer（NDO）is used to effectively estimate the time-varying wave disturbance and model uncertainty，and the exponential convergence property of the NDO is proved. Based on the UUV nominal model and the estimated disturbance state by the NDO，a backstepping sliding mode controller is designed using the backstepping method. By introducing a hyperbolic tangent function instead of a discontinuous switching function，the sliding mode approach law is improved and the chattering effect of the sliding mode is reduced. The stability of the system is proved using Lyapunov theory. Finally，the effectiveness and superiority of the control algorithm is verified through simulation experiments.