In order to improve the flight control accuracy of fixed wing UAV and reduce the influence of system dynamic coupling and external disturbance on the performance of flight control system of fixed wing UAV. This paper proposed a singular perturbation model of fixed wing UAV and designed a sliding mode control method based on disturbance observer. Firstly, the velocity and attitude of the fixed wing UAV are modeled based on the dynamics of action.Then we transform the dynamic model of fixed wing UAV into singular perturbation model, and then the singular perturbation model is decomposed to complete decoupling. Two reduced order uncoupled subsystems are obtained which is the fast subsystem with angular velocity and the slow subsystem with linear velocity and attitude.The anti disturbance sliding mode controller is designed for angular velocity loop,angle and attitude loop respectively. Finally, the feasibility and effectiveness of the sliding mode control method based on fast and slow decomposition are verified by Simulink simulation.