For the very long-span self-anchored suspension bridge crossing the navigable river basin, the conventional scaffolding construction method can’t be used to install the steel box girder of main span. Towards the above problem, the overall construction plan of “first cable-stayed, second suspension” was proposed. That is, the stiffening steel box girder was installed by making use of steel tower and stay cable to form the cable-stayed bridge. The main cable was installed and sling  was tensioned. Furthermore, the system transformation from cable-stayed bridge to self-anchored suspension bridge was carried out. In numerical analysis model of the complete system transformation, the coexistence of two independent cable support systems shall be considered. Using the unstressed state control method, all the cable units of the two models based on their unstressed length were used to rebuild the model for iterative calculation, so that the coupling state analysis model could be attained. The calculated results based on the recommended plan showed that after supplemental tensioning of the cable followed by the system transformation, the linear shape of main girder was significantly improved, the length of the extension bar of sling and the tensioning times were reduced, and the system transformation process was optimized. Based on numerical simulation and calculation, the changing rules of the sling and stay cable during system transformation were analyzed in the coexistent state of two cable support systems, the influences of tensioning siling on cable forces of adjacent and other nonadjacent silings were analyzed and the proportion of sources of cable forces after the completion of the bridge was also studied.