Abstract: It is very important to take the simulation analysis for the magnetic flux leakage testing both on pipeline and the defects on it. After comparing various magnetic flux leakage calculation methods, the finite element method (FEM) was taken as the analysis method in the research. The theoretical principle of magnetic flux leakage testing, the working mechanism and basic structure of magnetic flux leakage testing equipment were introduced, and also the FEM 3D module of defect magnetic flux leakage field in pipeline was built. Through the FEM and experiments simulation for multi magnetizing units, such conclusions were obtained as below: the magnetic field overlap from multi-magnetizing units will intense the magnetic flux leakage on the top of the defect, and also the number increase of the magnetizing units located at the central of the magnetizer will produce more intensive magnetic field than both side units of the magnetizer. The dimension of the defect will affect the distribution of the magnetic flux leakage filed. In a certain defect diameter size variation, there is a linear relationship between the intensity of the magnetic flux leakage field and the defect depth. No matter how thick the pipeline is, there is a similar varying trend for the axial component of the magnetic flux leakage field. Choosing a suitable lift-off distance will be helpful to get a better magnetic flux leakage field signal.