To reduce the influence of system dynamic coupling and external disturbance on the performance of flight control system of fixed-wing UAV thus improve its flight control accuracy, this paper proposes a singular perturbation model for fixed-wing UAV and then designs a sliding mode control approach based on disturbance observer.The velocities and attitudes of the fixed-wing UAV are modeled based on the dynamics of action.Then the dynamic model is transformed into a singular perturbation one and then decomposed to complete decoupling.Two reduced-order uncoupled subsystems are obtained, which is a fast subsystem with angular velocity as fast variable and a slow subsystem with linear velocity and attitude as slow variables.Then anti-disturbance sliding mode controllers are designed for angular velocity loop, and angle & attitude loop.Finally, the feasibility and effectiveness of the sliding mode control approach based on fast and slow decomposition are verified by Simulink simulation.