The utility model discloses a foldable rear supporting leg of a tunnel-through bridge girder erection machine. The foldable rear supporting leg comprises a folding control device, a rear fixed hinge, a front movable hinge, an outer cylindrical body, a lifting control device, a plug pin, an inner cylindrical body, three reaming supporting bases, a walking mechanism, an upper transverse linkage, a supporting rod, a middle transverse linkage, a plug pin device, a limiting device and a bridge girder erection machine main beam. The foldable rear supporting leg solves the problems that in the prior art, a space for a box girder to pass through can be made only when fixing bolts are manually disassembled and a rear supporting leg is moved out integrally, work efficiency is low and labor investment is large and has the advantages of being needless of other auxiliary equipment and capable of automatically and quickly making the space for the box girder to pass through, reducing operation difficulty and workload of workers and improving production efficiency.