A method of solving the initial structure of two-mirror reflecting system on the basis of the tmrd order aberration theory was proposed in this paper.In With this method.the parameters of initial structure could be calculated effectively and efficiently only by knowing the system focal length f,mirror separation d and the back focal length l'2.A remarkable advantage of this method was that the directly determined separation between two milTOrs was one of the conditions of solving the initial structure,thus it wound be more convenient and efficient when the geometrical dimensions of systems were limited.At the same time,some useful formulae were derived drawn,thus the obscurafion ratio and the magnification of secondary mirror could be calculated with the given parameters.Furthermore four typical two-mirror reflecting systems were discussed and a case was designed by this method.The results show that this method is comparatively accurate.作者简介：王占山(1963-),男,吉林东辽人,教授,博士,博士生导师,主要从事短波光学方面的工作.Emaii:wangzs@mail.tongji.edu.cn