Preload of angular contact ball bearing (ACBB) is extremely important to high speed spindle, which has a great influence on the spindle's dynamic and thermal characteristics, etc. In this paper, the preload source of ACBB; the main preload principles, methods and their advantages and disadvantages; research progress and development trend of preload device are discussed. On the basis, common criteria for computing and choosing optimal preload; influence of preload on dynamic and thermal properties of bearing and spindle system are discussed as well. The purposes of this paper are to clarify the research idea of preload, and provide a reference for accurately calculating the bearing stiffness under sophisticated conditions, precisely controlling dynamic parameters of high speed spindle, designing preload device with excellent dynamic performance, etc.