A bi-membrane axial adjusting mechanism is designed to meet the requirement of high precision and long distance in projection objective. The mechanism uses pressure for driving force. And optimization of different widths and thicknesses of diaphragms and width of the convex. It is optimized to gain the optimization parameters at certain pressure range. And then the frequency of mechanism is checked. The figure error of lens is analyzed to gain the Zernike coefficient when the mechanism is moving. The result indicates that the main figure error is power and sphere. The peak-to-valley (PV) of the low surface figure is better than 13.45 nm, and the root-mean-square (RMS) is better than 2.851 nm. The axial adjusting mechanism satisfies the requirement of lithograph projection objective.