Hydraulic support system is used as the primary mirror support of large telescope more and more. The cylin-der is the key technology of hydraulic support system and the axial stiffness is the key parameter of cylinder design. In this paper,a cylinder with rolling diaphragm was designed. The cylinders axial stiffness was analyzed with remaining air in cylinder being thought. The theoretical expression of axial stiffness of cylinder under this situation was got. The cyl-inders axial stiffness is found with remaining air being decreased. And the axial stiffness is increased with the pressure of Cylinder being enhanced. And the factor is got when the pressure of cylinder is set at 0.5MPa and the cylinders stiffness will be got to 10N/μm.%液压支撑系统作为大型望远镜的主镜支撑得到了越来越广泛的应用，而支撑液压缸是液压支撑系统的关键技术，液压缸的轴向刚度又是液压缸设计的关键参数。本文给出一款利用滚动薄膜作为密封件的液压缸设计，并在考虑存在残留空气的情况下，分析了液压缸轴向刚度，得到了此种情况下液压缸的轴向刚度理论表达。发现存在残留空气会显著降低液压缸的轴向刚度，而提高初始压强则会提高液压缸的轴向刚度，在实际应用中0.5MPa初始压强可以保证液压缸刚度达到10N/μm。
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