Abstract: The magnetic leakage field H yielded by a surface breaking discontinuity with any shape transversal section is analyzed from the concept of the watershed of magnetic charges. It is deduced that H∝(d-2kδ-2E), in which d is the maximum depth of the defect, k is a coefficient, δ is a small deflexion of the watershed of magnetic charges on the transversal section along the direction of the flaw depth, E is a constant depending on the watershed of magnetic charges. Thereby the following conclusions may be reached. ① Only when d, E are very small or when dW (the length of the discontinuity), there will be the relation, that H∝d. ② When dW, in a rough estimation H is independent of d, and only depending on W. ③ The figure of the transversal section of a surface breaking discontinuity can not be calculated out conversely from the magnitude and the distribution of its magnetic leakage field.