Abstract: On account of the lower resolution of the eddy current sensor in testing the slightly-changing signals, being directed against the defect measuring of this method in the study, a mutually complementary differential double parameter testing theory is proposed. Based on Ohm Law and the cylindrical conductors magnetic permeating formula, a mathematic model is established. It employs the general function and differential formula. Then the corresponding differential eddy current sensor and the testing means are designed. Experimental results demonstrate the validity of the theory.