Abstract: In order to improve the problem of poor accuracy of traditional method of calculating stress severity factor SSF in predicting the fatigue life of countersunk head connection structure, the paper researched the method of calculating stress severity factor SSF of countersunk head hole. Firstly, finite element analysis was used to obtain the contact stress region of the countersunk head hole and the extrusion distribution factor of the countersunk head connection structure, and a calculation method considering the influence of the countersunk head for maximum stress by nail load and extrusion distribution factor were proposed. Then, in combination with the traditional formula of the stress severity factor SSF, the calculation method of the stress severity factor SSF of the countersunk head hole was proposed. Finally, the calculation method was applied in the fatigue life prediction of the countersunk head connection structure of the strip plate. The results showed that, compared with the traditional calculation method, the proposed SSF calculation method had higher accuracy in predicting the fatigue life of countersunk head joint structure.