Abstract: During the using of metal parts in the circumstance of the earths magnetic field, the influence of geomagnetic direction on the magnetic distortion caused by stress concentration is still disputed. The relationship between the direction's change of feeble magnetic field (such as the earths magnetic field) and the magnetic distortion on the surface of metal parts was investigated, by simulating a ferromagnetic part rotating around the vertical direction in the geomagnetic field. The result showed that the range of the magnetic distortion was almost linear with the intension of the magnetic field in the direction of pull, it proved that in the geomagnetic field the magnetic distortion with the same pull mostly decided by the magnetic field in the direction of the pull force.