Abstract: The perpendicular component of magnetic leakage field yielded by a slot on the work piece surface Hz was analyzed by the simplified model of equivalent magnetic dipole in this paper. It was known that Hz must appear positive and negative maximum respectively at the spots where are (b+Δ) from the slot center in the crosswise (the direction of the slot width), in which Δ is a complex function of b and L, for any slot width 2b and lifting off L. So the transversal distance between positive and negative peaks of Hz, Szp-p=2(b+Δ), and Hz has not extreme value along with L change.