The longitudinal moment has an big influence on the stability and safety of the plane. During the research of Wing-hung and Low-HTP(horizontal tail plane) airplane the longitudinal moment stability declines along the lift increases. the HTP is an principal part which provides the longitudinal moment and the longitudinal characteristics of the whole plane is changing due to the HTP. The seven-hole probe is carried out to measure the dynamic press loss and down-wash angle of the HTP to find the reason. Results show that the loss of the dynamic press of the HTP at the angle which is before the maximum lift is bigger than original pressure. The test results are agreed with the calculated ones, and it could be used to optimize aerodynamic characteristics.