According to earthquake hazard, normal frame structures could hardly avoid failure of ‘Strong Beam and Weak Column’, weak layer and even progressive storey collapse under strong earthquakes, a main reason is the lack of second seismic fortification line in frame structure. Based on the design thought of multiple seismic fortification lines, reinforced concrete twin-frame structure is initiated in this paper. Major feature is increases of redundant constraints and load paths due to added outer limb-columns could take effect as the second seismic fortification line. Twin-frame structure could not only fulfill the demand of ‘three levels, two stages’ in seismic code, but also have been designed through the third stage of ‘resist progressive collapse’, using key member method to promote safety storage and deformability of outer limb-columns, using removal key member method to verify vertical ultimate load of frame beams in weak layer. Though twin-frame structure could not completely avoid plastic hinges in column ends, progressive storey collapse will not appear in this kind of structure. Twin-frame structure is a new structure system which could be applied in constituting new projects as well as strengthening and rebuilding existing projects, further research can be done on this structure system.