Flight loads and aerodynamic derivative analysis of a twin-tailboom aircraft was performed. The longitudinal aerodynamic derivative, the efficiency of each control surface and the load distribution were considered emphatically, as a function of the tailboom stiffness of longitudinal bend and the flight dynamic pressure. The tailboom stiffness of longitudinal bend has quite an effect on the longitudinal static stability, the efficiency and anti-efficiency speed of the elevator on the horizontal tail, even the load distribution of the horizontal tail. The stiffness of the tail section, especially the tailboom stiffness of longitudinal bend, uasually causes the weakness, as the aeroelastic analysis is processing on a twin-tailboom aircraft. The design standard of the tailboom stiffness must be controlled strictly.