Based on the theory of multi-body system，the spindle system of YK2045 CNC Spiral Bevel Gear Grinding Machine is investigated. It is treated as a coupled rigid-flexible multi-body system that made of rigid and elastic bodies linked together in a specific manner. A dynamic model is established. Computation of vibration characteristics is carried out, and the computation results based on the multi-body system theory and traditional lumped mass method are compared. Results show that the developed spindle system of Grinding Machine is always within a stable and reliable operating area.