Fastener is popularly used joint metal structure in aircraft. The flexibility of fastener is very important to analyze structure detail fatigue strength. Usually, many methods of fastener flexibility are all based on test. Lots of test should be taken and it would cost much. Study fastener flexibility by theory and get a method which can calculate fastener flexibility. In other words, this paper wants to find a method to instead of tests and time as more as possible. A formula which calculates fastener flexibility was found by using materiel mechanics and elastic mechanics. By comparing the method of this paper with the method which is from a famous aviation corporation or NACA report NACA_TN_1051, it proves that the method of this paper is consistent with them. And the method of this paper has considered more influence factors. So the method of this paper is useful in practice.