Abstract: In order to solve the defect quantification problem of pipeline in magnetic flux leakage inner detection for pipeline, a mathematical model of MFL field was built based on ampere circuital theorem, flux continuity theorem and cylindrical coordinate transformation. The rectangular defect data of magnetic flux leakage inner detection for pipeline obtained by finite element simulation analysis software were taken as research object, and the regression equations with defect size and magnetic flux density characteristics were established. The fitting of the equations for simulated data was realized with the least square curve fitting method. The error analysis indicated that axial linear equation with axial and radial linear equation could more accurately calculate the defect length, while axial and radial two fitting equations could accurately calculate the defect depth. The present research provided reliable analysis method for the defect quantification in magnetic flux leakage inner detection for pipeline.