Abstract: Due to the fact that the petroleum and petrochemical tank large fillet weld lies in a special position and easily generates the defects, there is no suitable ways to inspect large fillet weld. In this paper, the magnetic flux leakage (MFL) testing is applied to such large fillet weld testing. Model for MFL testing of large fillet weld was established by finite element method to simulate whether there is a level plate on the left side of the tank wall or not and whether defects exist in large fillet weld or not. When defects exist in weld toe and fillet weld heat affected zone, the curves representing distribution of the MFL field in the above case are obtained and furthermore, subsequent analysis is carried out and conclusion is obtained.