Abstract: In this paper,a new type of nondestructive testing technology based on the geomagnetic field is proposed for testing defects which are easily formed in the oil tank bottom plate such as corrosion, pitting, perforation. The artificial corrosion defects on plate were used as the research object. Two data processing methods of probability and Symlets wavelet decomposition were adopted for the analysis of the testing results, their advantages and disadvantages on the defect signal recognition were compared, and finally the reliability of detection method was verified by the in-site testing.