Abstract: This paper introduces the design principle of the step-flow detection and control in the physical model.It includes two parts:regulation of step-flow and the control of step-flow.According to the automatic regulative principle,how to reduce the transient error of step-flow is the technical key to this system.We adopt two independent step-flow detection and control units so that this technical difficulty is better solved.Because of making use of microcomputer control,the technical property and automaticity of this system are greatly increased.