Abstract: Core reactivity control and shutdown margin are provided by control rods. The actual physical position of control rod is monitored by rod position measurement system. After equipment type change of rod control and rod position measurement system in a nuclear power plant, an issue that fluctuation of measured position exists even with a stable physical position occurred during the system commissioning. Based on the study of rod position measurement principle and post-processing circuit characteristics, combined with the analysis of existing data, this paper determines that the root cause of the rod position fluctuation is that the power supply of the detector's primary coil is different in frequency and in phase, and the parameter setting method of rod position detector is also given. By data acquisition on site and development of calibration data processing software for rod position detectors, the rod position fluctuation problem of the power plant was solved. At the same time, the hardware-level optimization suggestions are put forward for the root cause of measured position fluctuation.