Abstract: Based on the daily variation of geomagnetic data recorded by fluxgate magnetometer GM4-2 at Hongshan geomagnetic observatory, this paper proposes a method for eliminating instrument orientation error, namely the correction method for magnetic axis zero drift S₀ of D element. First, based on the mea-surement theory of magnetic axis zero drift S₀, a rotation platform without magnetism has been trial-produced to measure the S₀， which is about 1024.7 nT for the GM4-2. Then, we correct the error of S₀ value in reorientation. Finally, the S₀ value is applied to both theoretical daily variation and daily variation recorded by the instrument after orientation. The results show that the magne-tic axis zero drift S₀ of D element is the main factor for the orientation error.The daily variation recorded by the instrument after orientation are consistent with the theoretical daily variation, thus eliminating the daily variation distortion caused by the orientation error, and reflecting the geomagnetic filed variation more realistically.