Pipeline magnetic flux leakage internal detection technology is internationally recognized as the most effective method for safety maintenance of long-distance oil and gas pipelines. To solve the problem of quantifying the internal detection signal of magnetic flux leakage in long-distance oil and gas pipelines under load, an improved three-dimensional magnetic charge mathematical model is formulated, which is based on the J-A theory. The influence of the magnetomechanical influence of pipeline defects on the magnetic flux leakage signal under internal pressure is analyzed. The influence law of the defect size on the magnetic flux leakage signal under the action of external load is studied, and the correctness of the theoretical model is evaluated through systematic experiments. Research results show that the internal pressure of the pipeline increases, the magnetization of the material decreases, and the radial and axial components of the magnetic flux leakage signal decrease exponentially. The peak value of the radial component and the maximum axial component of the magnetic flux leakage signal increase with the depth of the defect. The growth rate gradually decreases, and the eigenvalue changes by 6. 5% and 14. 7% , respectively. The peak growth rate of the radial component gradually decreases with the increase in length, and the axial maximum value decreases linearly. The eigenvalue changes by 21. 0% and 36. 8% , respectively. The axial component is more sensitive to changes in defect depth and length.