Abstract: In view of increasingly serious problem of power supply quality of coal mine power grid, an online monitoring system of coal mine voltage quality was designed by use of signal processing circuit, data acquisition unit and technology of virtual instrument of LabVIEW. Simulation and test results show that the system realizes online monitoring of power supply voltage quality of underground power grid, has functions of wave display in real time, measurement of parameters of voltage, frequency, harmonics, three-phase voltage unbalance factor, voltage fluctuation and flicker, data storage and data analysis, and is good in measurement accuracy and high in reliability with practice.