The invention discloses a diaphragm supporting cylinder for processing a large optical mirror surface, comprising a cylinder body and a piston assembly, wherein the cylinder body comprises an upper cylinder body and a lower cylinder body which are connected with each other; the piston assembly comprises an outer piston member and an inner piston member; piston cavities in the upper cylinder body and the lower cylinder body are T-shaped holes; a large diaphragm is arranged between contacting surfaces of the two inner piston members; a small diaphragm is arranged between the outer piston memberand the inner piston member; a cylinder upper sealing cavity is formed between the upper end of the large diaphragm and a small diaphragm installed on the upper cylinder body, a cylinder lower sealingcavity is formed between the lower end of the large diaphragm and the small diaphragm installed on the lower cylinder body; an upper cavity air inlet and an upper cavity air outlet passing through the cylinder upper sealing cavity are symmetrically arranged at the two sides of the upper cylinder body; and a lower cavity air inlet and an upper cavity air outlet passing through the cylinder lower sealing cavity are symmetrically arranged at the two sides of the lower cylinder body. The cylinder provided by the invention can quickly and effectively buffer the impact load of a cutter in the processing process of the optical main mirror and improve the processing precision of the optical mirror.