Extradosed cable-stayed bridges are hybrid structures,whose mechanic characteristic lies between continuous beam bridges and cable-stayed bridges.In recent years, extradosed cable-stayed bridges have rapidly developed in the foreign and domestic because of its economy and beauty.The paper tries to research the problems about the design theory for extradosed cable-stayed bridges roundly and deeply, then put forward a set of new design method.1、 The origin and development of extradosed cable-stayed bridges is firstly described in brief and then several problems by developing is pointed out.The name of the bridges in English and Chinese is discussed,and it is found out that cable angles and stress amplitude are essential differences between extradosed cable-stayed bridges and cable-stayed bridgesThe concept of cable/beam live-load ratio is introduced.Under uniform live load,the formulation of cable/beam live-load ratio for extradosed cable-stayed bridges with single-pylon,bi-pylon,multi-pylon is deduced and whose result is verified by that of FEM.Further research indicates that the cable/beam live-load ratio of 0.5 can be critical value between typical extradosed cable-stayed bridges and cable-stayed bridges or non typical extradosed cable-stayed bridges.lately the correlation for cable/beam live-load ratio and structure parameters is discussed. It is proved that the cable/beam live-load ratio reflects structure trait and is helpful for holding mechanic characteristic at concept design stage.2、 In order to study the whole rigidity of cable-stayed system bridges,the concept of nominal rigidity and pylon/beam rigidity ratio is introduced and whose formulation is deduced. Factors for influencing cable-stayed system rigidity are canvassed.And it is regard that pylon/beam rigidity ratio can be used to distinguish cable-stayed bridges and non typical extradosed cable-stayed bridges.3、 The feasible stress range of beam under dead load is determined by design strength subtracting the stress of live load and other action .By introducing the