The characteristics of magnetic field generated after the magnetization of pipeline girth weld defects were numerically simulated with finite element method based on magnetic flux leakage（MFL） principle in this paper. The magnetic flux density distribution curves were obtained in order to describe the features of leak magnetic field using COMSOL software. By changing the geometric parameters of weld reinforcement,pit,stagger and undercut,the magnetic field distribution of different defect types in different sizes is obtained. The results show that the magnetic field distribution of pipeline girth weld is thickened,and the radial and axial component values of magnetic flux density increase obviously with the increase of weld reinforcement. the weld and its central defect show the combined magnetic field distribution characteristics of both.The magnetic field distribution in the center of the weld is thinning,and the peak width of the axial and radial components of the magnetic flux density increases linearly with the increase of the pit diameter. With the increase of misalignment, the peak value of the flux de nsity distribution curve at the defect increases obviously;with the increase of undercut depth,the flux density at the undercut position decreases.