The invention provides a method for monitoring the expansion of fatigue crack in magnetic material by taking use of unusually changed magnetic signals, which pertains to the technical field of non-destructive detection. The method comprises the following procedures: a standard specimen is made with central crack of same material and heat treatment state as an object to be tested; after cycling preset times in fatigue testing, the central crack area of the standard specimen is scanned by a magnetic sensor with fixed liftoff, the normal component Hp(y) signal of the surface stray magnetic field of the scanned area is obtained; the length of the central crack 2a is measured; the max. value Delta Hp(y)max of the self unusually-changed magnetic signal generated by the fatigue crack is extracted as the detection threshold for the cycle numbers; the Delta Hp(y)max and 2a are linearly fit to get the Delta Hp(y)max-2a relationship formula; the object to be tested is inspected by the same liftoff with above magnetic sensor, the measured Delta Hp(y)smax is compared with the detection threshold; the expansion process of the fatigue crack is obtained according to Delta Hp(y)max-2a linear relationship. The invention is easy to operate, with accurate detection results, can monitor the expansion of crack dynamically under an unloaded state.