Abstract: The relation of the depth d of a rectangular through slot on the surface of a cuboid steel component and the magnetic leakage field H caused by the slot is analyzed by the model of magnetic dipole in the present paper. It is deduced, that 1) when d is very small, H∝d; 2) when d is larger, H∝(d-2kδ)(δ is a function of d, k is a coefficient); 3) when d is very large, on the contrary, H will decrease with the increasing of d. The inferences 1) and 2) have already been verified long ago by the classical experiments in magnetic particle inspection. And the inference 3) has never been seen in now available references, so it remains to be verified by experiment. Besides, five approximate analytical expressions between H and d are given.