Abstract: The magnetism-leaking method has high sensitivity in detecting the blemish of iron magnetism materials. This method can test different kinds of blemish which exists at the surface of the pole. However,because of the existence of greasy dirt (although has been cleaned),spot roughness,low straight line degree (although has been rectified) on the surface of the pole,the detecting probe and the pole do not contact tightly enough in the course of detection,which makes the detecting signals small,especially on fatigue cracks,it is not sensitive. On the base of manufacturing extended tube and the experiment platform of non-destructive testing,and through pre-fatigue crack and examination experiment,it concludes: the larger fatigue crack signal can be collected by extended detection pole,and the sensitivity improved by using of magnetism-leakig method.