During the equipment-system joint commissioning test of the rod control power supply system product of a nuclear power plant, failure occurred in the rectifier: for the rectifier operated with a pure resistive load, when the load current increased to about 20% of the rated current, the rectifier reported “synchronous fault” and stopped output. Considering the timeliness, cost and other aspects of the actual project, a four-stage second-order band pass filter with 50 Hz center frequency is designed for the synchronous sampling loop of the rectifier without changing the input side capacity. Through theoretical derivation, software simulation, hardware design and field simulation experiment, the purpose of normal operation of the sampling loop is effectively realized when the input side capacity is relatively small.