Implementing power quality analysis of the Experimental Advanced Superconducting Tokamak (EAST) power supply is great significance to the stable operation of the power system and the future development of nuclear fusion power stations. Due to the limited channels and fixed resolution of the existing monitoring equipment in the industry, it is difficult to cover the whole requirements of EAST power quality monitoring. And customized highly personalized equipment will be very expensive. Until now, the EAST power system lacks a multi-functional, high-precision power quality monitoring system. So that it cannot solve the impact fault caused by harmonics and inter-harmonics. Thus, a set of high-precision, multi-channel and real-time harmonic monitoring system with automatic trigger storage is established in this paper. Based on the tri-spectrum line interpolation method and the optional frequency-spectrum function, the spectral resolution can be improved to 1.25Hz. Meanwhile, the horizontal comparison of the data can more accurately locate and measure the characteristics of harmonics and inter-harmonics. Finally, the measurement data of the monitoring system is compared and verified by standard sources.