Position-limit protection equipment is the major control equipment on CNC machine, whose logic control method is the assurance of normal work of position-limit protection equipments. It will reduce the effect of position-limit protection or cause it malfunction, if we apply the logical control method not suitably. Current CNC machines are provided with a positive control limitation protection device, and then it is likely to happen malfunction even cause accidents due to its electric circuit errors or its open circuit. In order to solve this shortcoming, somebody has worked out a new method, which is called negative logic control. This paper introduces the new logic control method’s basic principle and its practical usage.