Abstract: In aircraft structure design, the safety factor is described with respect to the applied load, while the safety factor in the mechanics of materials is defined with respect to the material strength. It is difficult for beginners to distinguish the difference between them and to understand the connotation of the safety factor. In this paper, two definitions of the safety factor are compared and analyzed. The engineering implication of the load safety factor in the aircraft structure design is expounded. From analysis of the design value and the determination method of the safety factor for composite material structure, which are different from those for metal structure, the engineering significance of the load safety factor is made clear, which helps beginners to master the concept of the safety factor.