The determination of the safety factor requires consideration of the scatter of load and strength, material properties, defects in the manufacturing process, etc. Load and strength are the two main aspects, as they are the most basic design data. The current use of 1.5 as the safety factor value is a constant. This paper introduces the concept of safety factor, and describes the process of safety factor evaluation. The probability of failure-based method was used to evaluate the value of the safety factor of aircraft composite structures. The factors affecting the value of the safety factor, including external load, impact energy, residual strength and other variables, were statistically analyzed. The lack of damage detection probability calculation method was introduced, and the probability of failure calculation process and the safety factor evaluation process were given.