The main objective of this study is to investigate the effect of bearing preloads on the characteristics of spindle stiffness. Finite element model for spindle-bearing system is established enabling the variation of spindle stiffness calculations under different bearing preloads. On the other hand, the spindle stiffness may also be obtained from the experiment and its result may be utilized to validate the numerical calculations. The front-end section of spindle is acted by poise weights at different directions, and their corresponding deformations are measured through dial indicator. Three bearing preload conditions, i.e. light, medium and heavy preloads are selected, which are imposed on the bearing to investigate the spindle stiffness variation, respectively. In addition, the effects of the geometrical parallelism error at the end surfaces of spacer due to the manufacturing tolerance or some imperfection on the spindle stiffness are studied. Finally, the impact test on the spindle-bearing body is performed by hammer and the corresponding vibration signal on the spindle surfaces is acquired by accelerometer. The spindle stiffness may be reflected in the other manner through the signals processing by spectrum analysis.