The work aims to study the technology of smoothening the fused silica optical elements and explore the influence of different scanning velocities on the surface roughness of smoothed fused silica optical elements when CW CO2 laser is used to smoothen the fused silica optical elements under the fixed laser power and beam size. The study was carried out by simulation and experiment. A finite element model was used to simulate the laser processing model with a laser power of 25 W and an elliptical spot with a minor axis of 2 mm and a major axis of 3 mm. The temperature field and corresponding stable temperature of laser beam acting on the surface of fused silica under different scanning speeds were obtained. The accuracy of temperature simulation model was verified by fixed-point irradiation experiment, and single factor experiments with scanning speeds of 0.04 mm/s, 0.1 mm/s, 0.2 mm/s, 0.35 mm/s and 0.5 mm/s were carried out. Atomic force microscope was used to detect the surface roughness of fused silica before and after the smoothing experiment, and the relationship between roughness and laser scanning speed was obtained. When the laser power was 25 W, the stable temperature corresponding to the five scanning speeds was all distributed above the melting point. The surface roughness of the fused silica after smoothing at five scanning speed was significantly reduced, and the smoothing effect of reducing the initial surface roughness of the fused silica from 62.55 nm to 6.17 nm was obtained at a scanning speed of 0.1 mm/s. When the laser power and laser spot size are constant, there is an optimum scanning speed to reduce the roughness of the machined surface to the optical surface.