At the beginning of the 21st century, Su-tong and Stonecutters cable-stayed bridges were born into the world, which make the original span record of 890m up to 1088m. The increasing range of the span is so large that it is necessary to carry out a special research on structural system of the cable-stayed bridge with kilometer-scale span. Therefore, two cable-stayed bridges have been designed as the research objects, which come from the background of Su-tong cable-stayed bridge. And the main span is respectively 1308m and 1500m. In this paper, an analysis is carried out from the aspect of girder-tower relationship which comes from the concept of the structural system. In the static analysis, structural system, which is respectively floating, half floating and rigid, has been studied. And in the dynamic analysis, the girder-tower relationship has also been studied, which is respectively with no dampers, elastic links and viscous dampers. Based on the analysis results, structural system applicable for kilometer-scale cable-stayed bridges is summarized.