The invention relates to on-line monitoring device and method for voltage quality of an electric power system. The voltage quality of various voltage grades from 10kV to 1000kV of the electric power system can be on-line monitored in real time; stable state data of deviation, harmonic waves, voltage fluctuation and flicker, three-phase unbalance, and the like of the voltage is fully monitored; and transient data of transient overvoltage and temporary overvoltage are simultaneously monitored. The on-line monitoring device can acquire voltages in various voltage grades in real time from multiple paths at high speed by adopting a multi-channel high-speed acquiring device, an automatic recording device, a storage device and a display voltage quality monitoring device, and simultaneously record various quality data of 32 voltages. The on-line monitoring device can automatically start the storage and the record of overvoltage temporary waveforms and calculate overvoltage multiple and temporary frequency. The recording time can reach 5s. The device adopts a linux multi-task operating system and the whole software adopts a Web/Brower manner.