Abstract: The surface modification of 42CrMo steel was carried out by multi-channel electron beam repeated scanning.The effect of scanning electron beam process parameters on the surface roughness of 42CrMo steel was studied. The scanning electron beam process scheme was optimized and the changing rules of surface roughness, cross-section microhardness and microstructure of the modified layer were obtained. The results show that after the optimization treatment of scanning ring diameter 10 mm, scanning times 3 times, beam size 8 m A and scanning speed 4 mm/s, the microstructure of melting zone is acicular martensite and lath martensite, and the microstructure of HAZ is martensite, cementite and ferrite, and the surface roughness of 42CrMo steel is reduced from 3.200 μm to 0.5431 μm, which decreases by 83% compared with that of untreated samples. The surface microhardness of the sample can reach 855.1 HV, which is 2.7 times of the matrix hardness（321.2 HV）.With the increase of the cross-section depth, the cross-section microhardness reaches the second peak 841.5 HV in the fine grain heat affected zone, and then decreases gradually until it decreases to the matrix hardness.