Abstract: In this paper, the electronic control system of CNC grinding machine is respected as the research object, and we study the reliability index of it.Firstly, the reliability index system is introduced and the reliability index results are probabilistic.Next, the Edgeworth series method is used to calculate the reliability index of the electronic control system.Then, the traditional method (least square method)is used to calculate the reliability index of the electronic control system.Finally, the accuracy of the two methods in reliability function fitting and MTBF point estimation is compared with the sample observation value.The results show that Edgeworth series method is not only more accurate than traditional method, but also more convenient.The research results have been fed back to the manufacturer and approved, which is of great significance to improve the reliability level of CNC grinder.