The invention provides a self-adjusting mechanism and method for a gap of a disc brake for a vehicle, solves problems that existing disc brakes are short in service life and inaccurate in gap adjustment, and can effectively reduce eccentric wear phenomena of friction linings. The manner that two sides are driven by a middle shaft is adopted in the gap adjusting process; the mechanism comprises an eccentric rocker arm, a shifting pin, a shifting fork disc, an overload protection assembly, a sawtooth sleeve, a one-way bearing, a press disc, a compression spring, a gear shaft, a hinge pin, a reset spring, driven gears, driven shafts, push rods and a connecting bridge, wherein the shifting pin is mounted in the middle of an outer arc of the eccentric rocker arm in an interference manner, the shifting fork disc is limited by the gear shaft and the connecting bridge, the overload protection assembly is mounted in the shifting fork disc by the aid of the sawtooth sleeve and the compression spring, the one-way bearing is mounted in the sawtooth sleeve, a thrust bearing is mounted between the gear shaft and the reset spring, and the driven gears on two sides and the driven shafts are riveted integrally. The self-adjusting mechanism for the gap of the disc brake has the advantages of long service life and capability of realizing stable braking and accurate gap adjustment.