The purpose of the paper is to clarify the technological method of improving the surface roughness of 45 steel by scanning electron beam and the internal relation of technological parameters of electron beam polishing. Hot rolled air- cooled 45 steel is taken as the research object. By setting the parameters of programmable digital signal generator, the micro-melting polishing treatment is carried out on 45 steel samples, and the critical equilibrium relationship between the electron beam power and the power density required for melting is established. Numerical simulation is used to study the variation law of temperature field of samples under different process parameters. There is a nonlinear relationship between electron beam and workpiece moving speed in micro-melting state of 45 steel. When the electron beam current is 9.36 mA and the workpiece moving speed is 11 mm/s, the original milling scratches on the surface of 45 steel disappear completely, and the surface roughness decreases from 2.0 μm to 0.43 μm, with a decrease range of about 78.5%. Under some parameters, molten pits and wrinkles will appear on the surface, which is because the single scanning period is too long and energy accumulation leads to excessive melting of the surface. Milling scratches of 4# sample are still visible after electron beam treatment, because the single scanning period is only 3.6 s, and the surface energy loss is too fast, resulting in insufficient melting. Conclusion Scanning electron beam treatment on the surface of 45 steel can significantly reduce the surface roughness of the sample. By adjusting the process parameters to control the electron energy density and heat dissipation rate, the probability of molten pits and wrinkles can be reduced.