Abstract: A dual-mode scheme for fast digital control of servo system is presented in thispaper.To achieve fast and accurate positioning the bang-bang control is adopted forcoarse setting at the beginning when the deviation is relatively large;and the nonfluc-tuating least beat control is used for fine setting at a preset time towards the end when the deviation is relatively small.This paper includes determination of linear controlregion;description of the control mode switching conditions;establishment of the equations describing actual switching line,instruction changing line,and transfer function D(Z) of linear controller.The digital simulation results quite agree with theoretical computations.