A novel outer supporting structure for a rear auxiliary supporting device of a large-tonnage bridge erection machine comprises two outer supporting legs (1) which are supported on the left and right of a bridge erection machine main girder respectively. The outer supporting legs (1) comprise anchoring seats (3), outer columns (4), hoisting mechanisms (5), traveling wheel boxes (6), distribution beams (7), traveling rails (8), supporting hubs (9) and outer supporting oil cylinders (10). By means of the structure, the overall structure is compact and simple, the function is clear, all outer supporting leg mechanism work can be completed, and the direct effect of the device is that the outer supporting leg mechanism overall weight is reduced greatly, so that the outer supporting leg mechanism fabrication cost is reduced, the bridge erection machine girder feeding requirement can be met, the bridge erection machine bridge erection requirement can be met, and the requirement for the bridge face of the bridge erection machine during the maximum bearing force of the outer supporting legs is met.