In this paper, an adaptive dynamic programming-based sliding mode control scheme is established for the airspeed subsystem of fixed-wing unmanned aerial vehicle with uncertain disturbance. Taking advantage of strong robustness in sliding mode algorithm, a sliding mode control law is designed to compensate the influence of uncertain disturbance. Under the sliding mode control law, system trajectory of airspeed subsystem tends to the designed sliding manifold. Then, the tracking control problem of the airspeed subsystem under disturbance is transformed into an optimal control issue. An adaptive dynamic programming approach with actor-critic structure is utilized to obtain the nearly optimal control law with the nearly optimal performance. Finally, the simulation result is given to verify the effectiveness of the proposed control scheme.