The invention discloses a rear auxiliary support device for a novel large-tonnage bridging machine. The rear auxiliary support device includes an inner support leg and two outer support legs, wherein the inner support leg is supported at the middle part of a rear connection beam of a main beam of the bridging machine, and the two outer support legs are respectively supported on the left and right sides of the main beam of the bridging machine. The rear auxiliary support device for the novel large-tonnage bridging machine is compact and concise in whole structure, and definite din function, can complete all works of the outer support leg mechanism of the bridging machine, and achieve the most direct effect that the whole weight of the outer support leg mechanism is greatly reduced. The manufacture cost of the outer support leg mechanism is reduced. The requirement of the bridging machine on feeding beams can be met, and the requirement of the bridging machine on erecting the beams can also be met. The outer support legs also meet the beam surface requirement of the bridging machine with the maximum bearing capacity is ensured.