Incremental launching construction method for building main span of cable-stayed bridge

The invention discloses an incremental launching construction method for building a main span of a cable-stayed bridge. The method comprises the following steps: (S1) incrementally launching a main beam from an approach bridge to the main span through a slide way; (S2) when the main beam is incrementally launched to a position which passes through a main tower but not passes through the middle ofthe main span, building a process beam and a hanging rail system at two ends of the main tower, connecting the stay cable with the process beam, and longitudinally and transversely locking the processbeam and the main tower at the main tower, wherein the hanging rail system is connected with the main beam and the process beam; (S3) hanging the main beam below the process beam by virtue of the hanging rail system, and continuously carrying out incremental launching towards the middle of the main span; (S4) repeatedly carrying out the steps (S2) and (S3) until main spans at two ends of the mainbeam are closed and connec