Based on a designed vertical take-off and landing UAV with tilting ducted propeller and ducted coaxial counter-rotating propeller embedded inside the fuselage, the Fluent mesh is used to generate unstructured grids around the whole aircraft. The aerodynamic characteristics of the UAV are calculated with / without power, i.e. with or without ducted propeller slipstream. The aerodynamic characteristics including lift and drag coefficients of the UAV at a certain range of angles of attack under the conditions with / without power are obtained by CFD calculations, and compared with the wind tunnel test data. The calculated results show that the slipstream of the ducted propeller changes the aerodynamic characteristics of the whole aircraft. The slipstream causes the lift of the aircraft to increase at some angles of attack, which match the trend of the experimental data, and the drag increases significantly. Compared with no power, the lift and the drag of the whole aircraft increase, the stall angle of attack also increases. Under the state with power, the low pressure area on the upper surface of the wing is larger than that without power.