Abstract: This paper presents an approach based on numerical simulation for the magnetic flux leakage (MFL) field for the butt weld of the welding equipment such as the pressure vessel in order to study the weld defect. Comparative analysis of the leakage field characteristics was made for different welding processes (singlesided weld and doublesided weld) of having no defects, defects in the weld and defects in heataffected zone with finite element method (FEM) numerical simulation. Also, the laws of distribution of the MFL field are meanwhile studied when two rectangular defects are premade in the singlesided weld. Simulation results show that the doublesided weld has greater impact on the defect in the weld; also under the situation of coexistence of two defects the distance between the defects is the main factor to affect the MFL field distribution.