A simple technique is proposed for highly-efficient plane processing fully based on femtosecond laser beam shaping. The laser intensity distribution is transformed from a Gaussian to a donut shape. As the donut-shaped focus seems like a flat top from the side view, a plane with a high level of flatness is obtained directly by scanning once. By applying it to polishing experiments, the surface roughness can be improved significantly. The influence of scanning speed, laser pulse energy, and scanning times on the roughness is also discussed. Moreover, the scanning width can be flexibly controlled in a wide range.