Abstract: In order to reduce the cost of power door locks, a automobile door lock company changed the positioning method of the internal gear plate. However, it will cause the service life of door lock to be reduced. Standard QC/T 323—2007＂Motor vehicles—Door locks and door retention components＂stipulates that the service life of door locks should meet 100 000 times. In order to increase the service life of the power door lock to meet the standard, the fatigue test of the door lock was carried out first and it is found that the weak area is the tooth root of the gear plate in the door lock. Then with Using the finite element method, based on the material S-N curve and Miner linear fatigue damage accumulation theory, the fatigue analysis of the gear plate and the gear was carried out. The fatigue analysis results are close to the fatigue test results and the error is 8.3 %, indicating that the established fatigue analysis method of finite element is reasonable. Based on this method, the service life of the gear plate increased to more than 100 000 times by replacing the gear plate material. Finally, the fatigue test of the door lock with the new gear plate was carried out again and the test results show that the service life is more than 100 000 times, which meets the requirements. At the same time, based on the existing design scheme, each power door lock can save about 255 yuan in cost.