A floating type rear support leg of a large-gradient box girder bridge erecting machine at least comprises a connecting beam assembly and a walking assembly. The walking assembly is arranged at the lower end of the connecting beam assembly so as to enable the whole supporting leg to walk on rails. The connecting beam assembly provides supporting for a main beam of the bridge erecting machine, and comprises link beams, two columns and a lower cross beam, wherein the lower cross beam is transversely arranged, the lower ends of the two columns are connected to the two ends of the lower cross beam, and the lower edges of the outer ends of the link beams are connected with the upper ends of the columns. The walking assembly at least comprises two telescopic columns, two walking mechanisms and plug pin assemblies, wherein the walking mechanisms are supported on the rails, the upper ends of the walking mechanisms are connected to the lower ends of the corresponding telescopic columns, and the telescopic columns are connected with the lower cross beam through the plug pin assemblies. Accordingly, the floating type rear support leg is safe and reliable in design, easy and convenient to operate, and capable of erecting box girders for lines with the gradient being 3%, and the problems that in large-gradient box girder erecting, risks are high, investment is high, and the construction period is long are effectively solved.