The alternative current field measurement (ACFM) technique is a new non-destructive testing (NDT) method to evaluate the surface flaws, and in contract to other methods it has advantages of non-contacting and non-calibrating. Precise locating and crack length measurement can be easily achieved. But it is rather difficult to measure the crack depth with the complicated mathematic model. Its application is thus limited. On this ground, the swept frequency method is introduced to make up of this shortage. The penetration depth of eddy current that is theoretically different for different operational frequencies is the principle of crack depth measurement of the swept frequency method. By means of measuring the switching-frequency of the characteristic curve, the information of flaw depth is achieved by sweeping frequency. The paper introduced the physical model of ACFM and assayed the errors of measurement. Some methods were introduced to decrease the errors. The crack depth is proportional to the interval of the switching-frequency of flaw and of no-flaw, which is detected at the position of minimum value of Bz.