Abstract: A micro magnetic testing method in geomagnetic field environment was proposed to detect the defects existing in rare earth permanent magnet NdFeB materials. The automatic detecting device was designed to detect defects in permanent magnet NdFeB. A valid detecting algorithm was proposed to judging defects according to the curvilinear trend. Experiments were done through the micro magnetic detector of high accuracy and testing platform made by ourselves. The result show that for non-magnetizing NdFeB magnetic steel, the minimum defects is 1mm slot can be detected by the self-made device. This method has good engineering practical value.