2M virtual layers（Al2O3 coating，optical thickness is λ/2）are deposited on the surface of high reflective wave stacks，which reflectance is more than 99.999 7% and the center wave length is 635 nm，by ion beam sputtering method. Roughness，reflectance of all samples in different stages are measured and analyzed. The absorption of 2M Al2O3 coatings are calculated by optical coating theory. Effects of oxygen flow on absorption and the root-mean-square surface roughness（RMS）of Al2O3 coatings are discussed. The measured RMS value of all samples are better than 0.15 nm. The results show that 2MAl2O3 coatings have no obvious effect on RMS. The absorption of Al2O3 coatings is decreased with the increasing of oxygen flow at first stage，and then the absorption arrives its minimum，the absorption is increased with the increasing of oxygen flow at second stage. The extinction coefficient of 2M Al2O3 coatings lower than 5×10-6 at 635 nm can be prepared after optimizing process parameters.