In order to improve the reliability of the spindle system of CNC machine tools and find out the weak links of the spindle system,taking the spindle system of a heavy-duty CNC horizontal milling and boring machine as the research object, a fault tree with the spindle system failure as the top event was established. Due to the limitation of time and cost,it was difficult to obtain the accurate value of the failure rate of basic events. Therefore, combined with expert judgment and fuzzy set theory, the expert opinions were summarized into trapezoidal fuzzy numbers,and the occurrence probability of basic events was determined. Qualitative and quantitative analyses of fault tree were carried out,and the importance of basic events was calculated. Finally, the critical failure factors that affected the system were determined through the ranking results of the importance of the basic events.