The invention discloses a defect contour inversion method based on a magnetic flux leakage signal. The method comprises the steps of acquiring a to-be-solved target defect magnetic flux leakage signal; through edge detection, obtaining a defect opening contour shape which corresponds with the target defect magnetic flux leakage signal; obtaining N rectangles which are arranged in a direction whichis vertical with a magnetization direction; forming sub-defects of N cuboids; acquiring the defect magnetic flux leakage signal which corresponds with each sub-defect; performing displacement transformation operation on the defect magnetic flux leakage signal which corresponds with each sub-defect; obtaining a predicated defect magnetic flux leakage signal in a current predication depth sequence;obtaining an error between the predicated defect magnetic flux leakage signal and the target defect magnetic flux leakage signal, and furthermore when the error is higher than or equal with a presetthreshold, updating the current predication depth sequence, and returning to the S5 for performing continuous iteration, and otherwise, stopping iteration, wherein the current predication depth sequence is used as a final inversed defect depth sequence; and obtaining a final inversed defect contour. The defect contour inversion method greatly reduces calculation time and furthermore has advantagesof simple calculation model, high calculation speed, high precision, etc.