A one-way ratchet unit for a bicycle wheel has first and second meshing parts and an elastic member. The meshing parts are respectively configured on the bicycle wheel and the sprocket driving seat, their first and second one-way ratchets both comprise ratchet tips, ratchet roots and oblique guiding facets. The ratchet tips of the first or second one-way ratchet are configured with at least one wedge on partial positions, and the positions on the ratchet tips of the first or second one-way ratchet not forming the wedges form convex resisting edges. The ratchet roots of the second or first one-way ratchet are configured with wedge avoiding parts on partial positions, so that when the first and second one-way ratchets mesh each other and the resisting edges of the ratchet tips resist against the ratchet roots, said ratchet tips can be aligned and go into the wedge avoiding parts.