Abstract: According to the theory of magnetic dipole, the leakage field model of a high-pressure- tube with linear defect was established in vertical low-intensity field. Based on the spatial distribution of magnetic dipole, the equation of the normal component of leakage magnetic field on the tube surface under low-intensity field was derived. Under different longitudinal magnetization, the shape and characteristic of the zero value line of the normal component for the magnetic leakage field which was an important criterion for magnetic memory testing was analyzed.