On-orbit Assembling Space Telescope (OAST) technology, which was firstly presented by NASA around early 2000s, is one of the most feasible paths to the implement of 10m-class aperture large scale space telescope. Unlike traditional monolithic aperture telescope (like HST) or deployable space telescope (like JWST), OAST is modularly designed, modularly manufactured and modularly launched, the telescope modules are assembled and co-phased to diffraction limit in spatial environment.Considering the development of OAST involes multiple interfaces and new subsystems, on-orbit assembling and aligning procedure, the top-level design methods and considerations are quite different from monolithic space telescopes. this article introduces and discusses several op-level design consideration including optical and mechanical structure, on-orbit assembling and aligning method, and on-orbit testbed developing method.