Borehole seismic observation has the advantage of avoiding various types of surface interference,thereby significantly improving the signal-to-noise ratio of seismic observation signals. However,the installation azimuth of borehole broadband seismometers is arbitrary and must be corrected by comparing it to the installation azimuth of surface standard seismometers. This paper proposes a narrowband filtering optimal detection method for this purpose,which simultaneously calculates the relative azimuth and correlation coefficient in the north-south and east-west directions between the two seismometers. By selecting the results where the relative azimuth angle difference is within 1.2° and the correlation coefficient exceeds 0.995,an average is calculated,allowing for a quick determination of the relative installation azimuth between two broadband seismometers. The feasibility of the narrowband filtering optimal detection method is verified through a comparison of earthquake signals and Earth pulsation signals recorded at the Suixian Seismic Station in Henan Province and the Yichun Seismic Station in Inner Mongolia. The key advantage of this method is its ability to calculate the relative azimuth between two seismometers using just one night of recorded data. This makes it highly effective for azimuth correction during the installation of borehole broadband seismometers and can also be used to test alignment errors between surface seismometers.