The signal detecting circuit in static synchronous compensator (STATCOM) is the key to measure and control reactive power accurately. The design and implementation scheme of the signal detecting circuit of STATCOM based on DSP is proposed and the voltage synchronization signal and voltage and current instantaneous value detecting circuits are presented particularly. The experiment result shows that the developed circuit can be used not only to measure precisely the voltage crossing and the instantaneous value of voltage and current but also for other equipments which need to measure voltage，current and synchronous signal exactly，such as active power filter.