Magnetic flux leakage internal detection technology is one of the means for detecting defects in long-distance oil and gas pipelines. The identification of geometric features of defects is of great significance to the safety operation evaluation of pipeline. Based on the two-dimensional magnetic dipole model, a three-dimensional analytical model of the spatial distribution of the magnetic flux leakage field of the inner wall defects of the pipeline is established, and the variation law of the magnetic flux leakage field generated by the magnetic charge when the magnetization direction is perpendicular to the defect is studied. Based on the analytical model of the inner wall, a demagnetization factor is introduced to compensate the model. The three-dimensional analytical model of the defect leakage magnetic field on the outer wall of the pipeline is established, and the distribution characteristics of the defect magnetic flux leakage field on the outer wall of the pipeline are obtained. An experimental platform for magnetic flux leakage detection is built to verify the effectiveness of the model. The results show that the magnetic flux leakage field of the outer wall has a certain shielding effect, and the experimental results are in good agreement with the theoretical analysis. The model can effectively describe the spatial distribution characteristics of the magnetic flux leakage field on the inner and outer walls of the pipeline, which has certain engineering guiding significance for defect identification and quantitative evaluation.