Surface defects introduced by conventional mechanical processing methods can induce irreversible damage and reduce the service life of optics applied in high-power lasers. Compared to mechanical processing, laser polishing with moving beam spot is a noncontact processing method, which is able to form a defect-free surface. This work aims to explore the mechanism of forming a smooth, defect-free fused silica surface by high-power density laser polishing with coupled multiple beams. The underlying mechanisms of laser polishing was revealed by numerical simulations and the theoretical results were verified by experiments. The simulated polishing depth and machined surface morphology were in close agreement with the experimental results. To obtain the optimized polishing quality, the effects of laser polishing parameters (e.g. overlap rate, pulse width and polishing times) on the polishing quality were experimentally investigated. It was found that the processing efficiency of fused silica materials by carbon dioxide (CO₂) laser polishing could reach 8.68 mm² s^-1, and the surface roughness (Ra) was better than 25 nm. Besides, the cracks on pristine fused silica surfaces introduced by initial grinding process were completely removed by laser polishing to achieve a defect-free surface. The maximum laser polishing rate can reach 3.88 μm s^-1, much higher than that of the traditional mechanical polishing methods. The rapid CO₂ laser polishing can effectively achieve smooth, defect-free surface, which is of great significance to improve the surface quality of fused silica optics applied in high-power laser facilities.