Abstract: A new online dynamic balancing method is introduced based on influencing coefficient method of discal work machined on the CNC vertical turning machine.It considers the spindle system as a rotor system,and calculates and saves the amplitude and phase influence coefficients which are got from the testing unbalance and the vibration response of the spindle system.When the work has been done,the primitive unbalance by means of the detection and control system without halting the machine is obtained,and then dynamic balancing operation is executed according to the practical situation.Finally,the experiment shows that this new method has the feature of high balancing precision,high feasibility and easy operation.