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Underground ferromagnetic pipe can be magneticed by earth magnetic field, superposition of magneticed field and earth magnetic field produce magnetic abnormal. By pipe geometry modeling and Magnetic dipole reconstruction we can do Magnetic abnormal forward analysis. Initially, approximate cube elements are used to reconstruct pipe geometric model and output volume and center coordinates of elements. Then cube elements are seen as magnetic dipole and magnetic dipole reconstruction is used to forward pipe magnetic abnormal. At last research the influence of Distance-to-Diameter ratio, magnetic inclination, magnetic declination, geomagnetic field and pipeline’s susceptibility on pipe magnetic abnormal forward calculation. The results show that:when Distance-to-Diameter ratio is bigger than 6, magnetic abnormal of measurement points on the same sphere are very close and pipe magnetic abnormal is not affected by magnetic inclination, magnetic declination, and pipeline’s susceptibility; pipe magnetic abnormal increases linearly with the increase of geomagnetic field. This result can lay a theory foundation for high precision underground pipeline detection equipment.