Abstract: Firing synchronization is the key for thyristor rectifier control in the EAST polarity field converter power supply system. The converter-transformed second ary voltage signal was adopted as the synchronizing signal to improve the control precision. The state variable filter was used to filter the harmonics and the noise of the synchronizing signal. According to the requirements of the synchronizing signal, the filter design criteria were proposed. The analysis and the parameter calculation of the state variable filter were also made. The real-time simulation and the experiments on the EAST polarity field converter power supply system prove that the designed filter can extract precise synchronization signals.