In view of the problem that the existing APF(active power filter) has less data storage, weak data processing ability and poor ability of remote monitoring, the signal acquisition, processing, transmission, display and so on of the APF in the operation are studied. The design method of online monitoring system is summarized. The realtime monitoring system for APF operation is designed. This system adopts the lowpower STM32F103RBT6 chip as the signal collection and processing controller, using STM32 development board peripheral function module for data transmission, storage, and using the advanced standard communication interface on development board to realize RS485 remote communication. Thus the problems that APF data storage capacity is small and remote monitoring ability is weak are fundamentally overcome. The experimental simulation results show that this system has greatly strengthened the ability of APF data acquisition and processing, significantly enhances the data storage and remote monitoring effect is good. The system operation is simple and has low cost, thus it has good application and popularization value.