The invention discloses an installing method of reinforced concrete bowstring rod arch bridge arch ribs and a hanging auxiliary bridge. The method and the hanging auxiliary bridge are used for installation of the reinforced concrete bowstring rod arch bridge arch ribs during the first-beam and second-arch construction and the in-site casting of bowstring rods. According to the method, an arch rib hanging auxiliary bridge is built in a riverway along the ridge direction of a bowstring rod arch bridge, the arch rib hanging auxiliary bridge comprises a reinforced concrete carrying table, a support pier and a hanging table top. A bowstring rod cast-in-place support frame and an arch rib installing support frame are built on the arch rib hanging auxiliary bridge, and the bowstring rod in-site casting is carried out by using the bowstring rod cast-in-place support frame; through the arch rib installing support frame, a truck crane is used for installing arch ribs of the bowstring rod arch bridge on the arch rib hanging auxiliary bridge. The method and the hanging auxiliary bridge provided by the invention are applicable to navigable riverways, are also applicable to unnavigable riverways, and have the advantages that the construction is convenient, the greatest difficult problems of arch rib installation can be solved, meanwhile, the problems of in-site casting of the bowstring rod, the deck construction of the arch bridge and the like can be solved, the building cost of the bowstring rod arch bridge is greatly reduced, the construction safety is improved, and the building application range of the bowstring rod arch bridge is expanded.