For the mirror with traditional honeycomb sandwich structure, due to the existence of grid effect in the processing, the thickness of the mirror panel and the honeycomb size are correlated with each other, which seriously affects the lightweight design of the mirror. Aiming at the ultra-light mirror with honeycomb sandwich structure, an inflatable balanced processing method to reduce the grid effect was proposed. By using the control variable method to design test, the changes of grid effect under normal processing and inflatable balanced processing were compared. The experimental results show that when the mirror surface precision RMS reached more than 1/10λ （λ=632.8 nm）, there is an obvious grid effect in the normal processing without inflation, but not in the inflatable balanced processing. It could be seen that inflating the inside of the mirror could effectively balance the processing pressure and make the deformation of the reinforced area and the non-reinforced area tend to be the same during processing, so as to effectively reduce the grid effect.