Abstract: After machine processing, the rotor shaft was tested by magnetic particle, founding that the presence of a number of magnetic marks indications along the axie.In order to study the property and the produced reason of the magnetic scar indications,analysising to the reason of the magnetic mark by using magnetic particle testing,low magnification inspection,microstructure observation and electron probe micro-composition analysis,etc. The results showed that the microstructrure in the light bar area which the part of magnetic marks indications corresponding to is massive acicular martersite and remained austenite, the Cr,Mo,Mn,Ni alloying element contents were higher than normal area. So the magnetic mark indications of the rotor shaft along the axial distribution is the austenite,it was caused by the Ms reduction because of segregation of elements.