Abstract: This paper deals with the fault data of the spindle system of a series of CNC grinding machines in Beier Machine Tool Works, draws the scatter diagram of the probability density function of the spindle system, and preliminarily assumes the function model of the system. Then the least square method is used to estimate the unknown parameters, chi square test and correlation coefficient method are used to verify the goodness of fit, and finally the distribution type of the spindle system is determined as exponential distribution. The mean time between failures (MTBF), mean time to repair (MTTR) and inherent availability (A) of the spindle system are calculated according to the distribution type, and the reliability of the spindle system is evaluated. The result has been feed back to the manufacturer and approved, which is of great significance to the reliability improvement of CNC grinder.