Abstract: Magnetic flux leakage(MFL) technology was seldom applied on testing for the butt weld of steel plate, in view of the characteristics of the ferromagnetic weld, butt welds cracks of the steel plate with MFL method were investigated according to the cracks that were relatively more sensitive to welded structure. 3D finite element method(FEM) model of the weld was established by using FEM; the MFL field distribution was analyzed comparatively when the crack exists in the different regions of the weld; 3D FEM models of different weld reinforcement with the same size cracks were established, also the effect of the detection rate of cracks because of weld reinforcement was analyzed; the laws were obtained representing distribution of the component peak of the magnetic induction to the change of the depth of the crack, and the curves were obtained representing distribution of the MFL field under 3mm conditions of weld reinforcement. The outcome indicates that MFL method is suitable for testing study of butt welds defect of the steel plate; the detection rate of defects is getting higher as the weld reinforcement is becoming smaller; the trends are increasing with the component peak of the magnetic induction of the MFL field to the change of the depth of the crack.