Surface roughness of part was vital to its application.40Cr material was irradiated by large scale pulse electron beam with orthogonal experiment and the surface roughness was checked.The results indicate that the surface roughness of 40Cr is changed by different electron beam parameters.Surface roughness of part increases or decreases in terms of electron beam parameter.The samples with high and low surface roughness are tested with electron beam respectively.The results show that the surface roughness decreases with irradiation numbers increasing in high original surface roughness,contrary surface roughness increases with irradiation numbers increasing in low original surface roughness.It is analysed that the surface roughness is influenced in two opposite sidedness when it is modified on the surface with pulsed electron beam.