The invention comprises two transmission rods, two auxiliary supporting rods, two guiding rods and two pedals. A first pivoting portion capable of coupled at corresponding crank free ends is arranged between the front ends and the rear ends of the transmission rods, the rear ends are positioned on the outer side of a rear wheel of the bicycle, guiding holes are concavely formed in the end faces of the rear ends, and the pedals are mounted at the front ends of the transmission rods; the auxiliary supporting rods are fixed disposed at two ends of a rear wheel shaft of the bicycle and extend in a direction facing the rear side of the rear wheel of the bicycle to be coupled with the guiding rods, and the guiding rods can be symmetrically matched with the guiding holes of the transmission rods slidably; when two feet stamp the pedals, the transmission rods can swing by taking the first pivoting portion as a supporting point, and the guiding rods can extend and contract along the guiding holes and can swing by taking a second pivoting portion and a third pivoting portion as supporting points, so that the pedals can drive the bicycle to advance in a laborsaving manner.