Magnetic flux leakage (MFL) technology is suitable for weld crack, porosity and other defects testing. Due to the particularity of weld, magnetization paths and MFL signals under different magnetization directions are different, and adopting appropriate magnetization direction can stimulate stronger MFL signals, thus improving the detection rate of small size defects. Ansys FEM simulation software was used to calculate the MFL signals of weld defects, and the strength and components of the MFL signals of circular holes and longitudinal rectangular slots defects when the magnetization direction was perpendicular or parallel to the weld seams were respectively analyzed and compared quantitatively. On this basis, the variation rules of the MFL signals in different magnetization directions were discussed. A MFL testing platform for welding seams was established for experimental verification. The experimental results show that the magnetic flux leakage signal amplitude of circular flaw is only 18.6% of that of parallel magnetization, and the magnetic flux leakage signal amplitude of longitudinal flaw is only 9.2% to 29.3% of that of vertical magnetization.