This paper analyzes the power quality measuring principles according to the national power quality indicator. And a power quality monitoring system based on LabVIEW has been designed. Five indexes are introduced simply in the paper. They are voltage deviation, frequency deviation, three-phase unbalance degree, power harmonic and voltage fluctuation and flicker. Virtual instrument technology is used in the system. Through the combination of software and hardware and with the help of software simulation, the system gets signal data and analyses the power quality. The main program interface has been shown. Simulation showed that the system is feasible. Compared with the traditional instruments, the system has a friendly man-machine interface and strong processing capacity. It can be operated simply and extended conveniently.