A fine adjusting structure for bicycle disc-brake system is disclosed. The disc-brake system comprises a securing seat mounted on to the fork of the bicycle frame and the securing seat being locked with a braking main body having a driving arm, the wheel of the bicycle being mounted with brake disc for the gripping of the braking main body, characterized in that the top and bottom end of the securing seat are respectively protruded out a locking section toward the braking main body and the securing seat is substantially a "C" shaped structure, the top and bottom of the locking section are formed into elongated hole which passes through from top to bottom, the long axis of the elongated hole is extended in the same direction of the wheel axle; and the top and bottom edge of the braking main body are respectively formed into screw holes corresponding to the elongated hole of the locking section of the securing seat.