The static stiffness of a machining center's spindle was studied based on Workbench software. The static stiffness of the spindle was optimized on the basis of statics analysis, with the spindle quality as constraint condition. The six sigma reliability analysis of the optimized spindle was carried out using the ideas of probability and statistics, which considered the random distribution of geometrical dimensions, external loads, material characteristics and bearing stiffness. The results show that the spindle stiffness could satisfy the using requirement and its stiffness increased by 17.5% after optimization. Reliability sensitivity analysis showe that the random variation of external loads, bearing stiffness and the spindle diameter has a great influence on the spindle stiffness. The spindle is safe and reliable after optimization with its stiffness reliability is 100%.