Abstract: Described in the paper is the formation of the magnetic leakage field during the magnetic particle testing process.It is identified via the analysis that the factors influencing the magnetic leakage field are mainly related to the degree of magnetization of the workpiece,and the position,the direction and the depth-width ratio of the defect;the magnetic leakage field is getting bigger and bigger along with the increase of the defect depth;when the defect main plain is perpendicular to the magnetic field,the magnetic field becomes mostly strong;and the magnetic leakage field is also getting bigger and bigger along with the increase of the defect depth-width ratio.The formation principle of the rotating magnetic field is elaborated.And the liability of detecting defects at different directions with the rotating magnetic field is verified by means of applying the rotating magnetic field to the magnetic particle detection of steel pipes and steel bars.