The longitudinal aerodynamic characteristics of transport aircraft at the large flap angle are studied by the numerical method, and the flow mechanisms for such flows are analyzed. The results show that the limited separate flow of horizontal tail at the la rge flap angle causes the dis-original elevation oscillation, and the blended area of tail and body has large adverse pressure gradient, where exists the partial separated flow. The flow characteristics in this area causes the abnormal elevation oscillation for the aircraft. The results are also validated by experiments and flight tests.