With the analysis of traditional power quality monitor system, this paper introduces the system built by virtual instrument technology. From the power quality indicators and the measuring principles, we designed a data acquisition hardware based on the USB bus and developed the monitoring system applications using LabVIEW software. Comparing with conventional power quality monitoring system, the system overcomes the weaknesses of huge analog test devices. In this system, the intuitive interface makes operation simple with rapid response and small errors. In addition, statements and automatic alarm function is added to the software programming, which will be a great practical significance of the realization of automated monitoring.