Abstract: With the development and construction of urbanization and the increase of population density in Hohhot, the Hohhot subway was officially put into operation in January 2020, which immediately caused serious high-frequency noise interference to the original waveform produced by Hohhot geomagnetic station. As a result, the effective utilization rate of geomagnetic data is greatly reduced. Through the analysis of the basic principle of the subway power supply system, the interference mechanism caused by the geomagnetic station is analyzed, and the basic characteristics of the interference are summarized. We also analyzed the frequency band range and spectrum characteristics of the subway interference by the method of wavelet transform. The energy density distribution in time-frequency domain is obtained. On this basis, BD wavelet decomposition method is applied to filter the interference frequency band, so as to effectively improve the quality of geomagnetic observation.