Abstract: Simulation analysis and test of the effect of ferromagnetic filler on the magnetic flux leakage detection of tank floor defect were carried out in this paper. By using Belzona as ferromagnetic filler, the finite element model of the bottom plate filled with different size defects of Belzona was hence established, and the spatial distribution and variation law of the leakage magnetic field of the defects were obtained. The laboratory research shows that the defects filled with Belzona can be detected by magnetic flux leakage testing method, and the change rule of magnetic flux leakage is the same as that of unfilled Belzona, but the magnet flux leakage intensity is less than that of unfilled Belzona; for defects of different volumes, with the increase of defect volume, the effect of Belzona on the test result shall show a tendency to increase.