Abstract: In allusion to the problem that traditional reactive compensation method could cause over-voltage and over-current which seriously disserves power quality, the paper proposed a design scheme of reactive power compensation system based on phase-controlled switching technology, and introduced principle of phase-controlled switching technology, overall structural design and software design of the system. For two key problems of the system, the paper proposed an extraction method of voltage zero-crossing signal and a method of using BP neural network algorithm to predict time of switching off or switching on. The system switches capacity while the voltage or current signal at zero crossing time, and realizes reactive power compensation of the grid. The tests showed that the system can accurately and real-timely compensate reactive power required by grid and improve power factor.