The characteristics of different spindle systems were analyzed and compared;and then the direct-connected spindle system was determined to be used in high-precision CNC horizontal jig boring machine.Considering the requirements for machining process of machine tools,the maximum cutting mechanical parameters were calculated under different processing modes.Based on the results,the selection of main motor and spindle bearings was completed.According to the function and accuracy requirements of designed spindle system,the complete mechanical structure was obtained.It shows that the designed direct-connected spindle system can meet the requirements of accuracy and performance level through prototype test and application verification.