Abstract: In view of problems of high energy consumption, many fault points and bad reliability caused by TKD speed-regulation mode used by electrical control system of hoist of old auxiliary shaft of a Coal Mine, the paper proposed a reform scheme of medium-voltage frequency-conversion speed-regulation system with double three-level based on PLC. It introduced hardware structure, working principle and functions of high-low voltage distribution system, back-to-back medium-voltage frequency converter with double three-level, multi-PLC network control system and upper computer monitoring system. The actual application shows that the reformed system has low energy consumption, stable and reliable running.