The magnetic interference will destroy the attitude information contained in the magnetic sensor output, and thus reduce the attitude estimation accuracy of the fusion estimation algorithms based on magnetic, angular rate and gravity (MARG) measurements. In order to improve the antimagnetic interference ability of attitude estimation, a method of correcting the magnetic sensor output under the condition of static or small motion acceleration is studied. In this method, the normalized magnetic sensor output vector is corrected by the gravity acceleration measurements. The correction method is based on the principle of a constant angle between the geomagnetic field and the gravity acceleration and the principle of minimum correction angle. The fusion model and the correction principle are analyzed, and the correction equations and formulas are deduced. The effectiveness of the correction method is verified by static and dynamic experiments. Experimental results show that the RMSE of pitch and roll estimation can be reduced by 26°and 16° respectively. In addition, the correction does not change the fusion process, and thus is compatible with other improvement measures.