In order to solve effectively many shortcoming of traditional load design, a threesection method, namely segment stiffness, segment mass and segment aerodynamics, for the load design of aircraft was presented. The method approximates the actual mass distribution and aerodynamic load distribution of the aircraft. For a simplified aircraft, the modal parameters and the section load were calculated by using the threesection method, the theoretical method and the masspartition method. The results show, the threesection method and the theoretical method are consistent with the model parameters and the calculation principle, which largely can be considered as a method, so their modal parameters and the section load are identical. The load of the left and right section of the masspartition method is pretty inconsistent, and it is not in accordance with the actual load condition. In a word, the threesection method can get a more real and reasonable section load distribution, and the engineering application is simple, reasonable and reliable. At the same time, the method can reduce the design difficulty of missile load and structure.