The aerodynamic characteristics of a tandem wing aircraft are complex and difficult to predict due to the interaction between front and rear wings and fuselage. In this paper, five aerodynamic configurations were designed for a tandem wing aircraft with the vertical distance between the front and rear wings as a variable, and the CFD method was used for numerical simulation. Results of computation show that the values of gap have significant effects on aerodynamic characteristics, including location of pressure center/ aerodynamic center, longitudinal stability and lift-drag ratio. Increment of gap causes rise of lift-drag ratio and rearward movement of aerodynamic center. At same gap, low fore wing configuration shows higher lift-drag ratio, and high fore wing configuration appears to be more stable in longitudinal direction.