Abstract: In this paper we analyzed the increased noise in the observation data of FHD proton vector magnetometer at Suqian station from April 28, 2020. After we checked and analyzed the instrument parameter settings, observation environment, instrument and operation conditions, the failure of FHD instrument is confirmed. The capacitor in the instrument compensation current circuit was broken down due to aging, and we solved the problem by replacing the faulty capacitor. After the repairmen, the calculated background noise of magnetic declination <i<D</i<, horizontal component <i<H</i< and vertical component <i<Z</i< decreased from 1.19′, 1.45 nT, 1.58 nT to 0.82′, 0.32 nT, 0.50 nT, and the noise quality of each component had been significantly improved. Through the inspection, detection and judgment of geomagnetic instruments and external observation environmental problems, we summarize the reasons for the noise fault. This paper provides a scientific and effective detection method for geomagnetic stations to find and analyze the main reason for the data noise problems.