Abstract: It is well known that in the petroleum and petrochemical industries the fault in tank fillet weld is difficult to detect. Present work aimed to use the magnetic flux leakage detection technology to detect cracks in tank fillet weld. Finite element analysis software was then applied to establish a model of fillet weld crack defects, to obtain the spatial distribution of the magnetic field of the fillet weld cracks, and to extract out the magnetic flux leakage signals of the fillet weld cracks. The affecting factors of the fillet weld crack defect on magnetic flux leakage signals were analyzed. On this basis, experiments on the tank fillet weld cracks by using magnetic flux leakage detection were conducted, and the magnetic flux leakage signals obtained were analyzed. Results by the finite element analysis are consistent with the results of magnetic flux leakage signal analyzed, and it proves the feasibility and validity of the detection system established.