In this paper,the stress field and the stress- magnetic field theory for a crack domain are used to explain the principle of magnetic memory testing. The magnetic dipole model is employed to the quantitative research of crack length. The magnetic memory testing for different metal pipe cracks is made and the characteristics of magnetic memory signal are analyzed. It is shown that on the crack tip stress concentration appears,the magnetic field strength and it’s normal component reach the maximum. However,near the middle of the crack,the tangential component of the magnetic field intensity attends the crest and the normal component has zero-crossing feature. The crack length is approximately proportional to two parameters,the distance between the adjacent peaks of the tangential signal gradient,the distance between the peak and valley of the normal component of the signal. Based on the experiment,a calculation formula with good accuracy is suggested to determine the crack length and is proved by the experiment result.