The Lift Fan UAV in flying wing configuration not only has the advantages of the flying wing, but also can be capable of short distance/ vertical takeoff and landing(S/VTOL). To study its aerodynamic performance is the foundation for building the dynamic of flight. In this paper, the Lift Fan UAV’ aerodynamic was studied in transition stage, and analyzed lift, drag and moment with changes of the velocity and angle of attack. Aiming at the special velocity, the reason was explained by the streamline. In addition, some method to release the fluid separation was put forward. According to the result, it can be seen that the lift, drag, and up nose moment increase with the increasing of velocity. At the same velocity, by the angle of attack going up, the lift is going up and the outer wing is the main source to get the lift. The drag decrease first and increase finally. In comparison with the normal steady flight, the up nose moment is much higher. Besides, these two medications to improve the fluid separation are simple and effective.