Many factors could lead to deviation of focal plane of a space-borne camera from its ideal position and thus on-orbit focusing are indispensable to capture satisfactory images of space targets. Among all typical focusing techniques, changing the position of focal plane directly through motor-driven worm and gear is the simplest one, but two drawbacks are obvious. First, mechanical movement is slow but the space targets usually move very fast. In this case, it is highly probable that focus adjusting is always lagging. Second, the targets especially the non-cooperative ones may appear anywhere and working distance of defocus compensation should be large enough which makes the focusing assembly much heavier. Factually, most large aperture space-borne cameras are all-reflective or all-reflective having lens correctors, therefore by changing the interval between the primary and secondary mirror or by changing intervals within lens correctors defocus could be compensated. Although the sensitiveness is improved, moving elements are still needed indicating underlying lagging. Therefore in this manuscript, a new focus adjusting method is proposed. By changing the secondary mirror into a variable curvature mirror (VCM), the defocus compensation could be realized by varying the curvature radius of VCM. One prototype space-borne optical camera whose focal length is 6000mm and aperture is 600mm is used to verify the method. Our research demonstrates that the VCM based focus adjusting is not only very sensitive but also suitable for very severe defocus. Specifically, only a slight saggitus variation of less than 4um could compensate amazing defocus of about 4mm while maintaining good linearity between the saggitus variation of VCM and defocus, which proves the potential of this focus adjusting method.