Abstract: In this paper, the performance and detection principle of MFL technology, the structure, technical property and detection process of LCTS-1 leak detector used to detect plate corrosion for the vertical aboveground tank are described. The case analysis on the detection curve is made. The practical leak detection results show that this kind of detector not only finds out corrosion defects on the surface of tank plate, but also detects the corrosion defects on the reverse surface of bottom plate and top plate of oil tank.