Abstract: Propeller slipstream has an important effect on the airplane's aerodynamic characteristics. The wind tunnel test indicates that under the condition of large pulling force, the airplane under study has longitudinal static instability at small angle of attack which affects the safety of aviation seriously. The test research on slipstream shows that the longitudinal static instability at small angle of attack is mainly caused by the horizontal tail doing the traverse progress in the slipstream, which induces the horizontal tail's efficiency to change obviously. With the layout characteristics under consideration, it is advised to reduce the height of the horizontal tail to reduce the power's adverse effects. The test proves that reducing the height of the horizontal tail can improve the longitudinal static margin at small angle of attack obviously, and enlarging the range of the longitudinal static stability.