Abstract: The bombardment of advanced energy-carrying electron beams can quickly increase the surface temperature of workpieces. The surface hardening treatment on a quenched-tempered 42 CrMo steel was carried out by changing the electron beam current（25 m A-50 m A） and accurately controlling the surface heating temperature to avoid surface melting of the steel.The microstructure, phase structure and hardness of surface hardened layers were detected and analyzed. The results show that both the area and the maximum depth of the surface hardening zone of 42 CrMo steel exhibit a non-linear relationship with the beam current（as well as the density of electron beam energy）. The hardening effect is significantly improved when the beam current increases over 36.7 m A. Under the beam current of 50 m A, the maximum depth of surface hardening zone reaches the extreme value of 1.1 mm and the maximum hardness reaches 705 HV0.3. The width of the heat-affected transition layer between the hardened zone and the matrix is only 50-60 μm. The hardness is about 615 HV0.3, which is much higher than that of the quenched and tempered matrix.