An automatic compensation device for a drum free gap for linking up a brake cable and a brake rocker arm assembly is disclosed. The automatic compensation device comprises a fixing seat, a casing, a connecting sliding seat and a rotating guiding mechanism; the casing is pivotally mounted on the fixing seat; the connecting sliding seat is freely movable in an inner diameter chamber of the casing, aone-way bearing is embedded on one end surface, and a screw is adjusted by the nesting of the one-way bearing. The other end surface of the connecting sliding seat has a connecting portion connectingthe brake wires; the rotation guiding mechanism is used for arranging guiding pins and the guiding grooves including the straight groove and the inclined groove that are correspondingly nested on each other on the casing and the connecting sliding seat respectively; the automatic compensating device rotates the connecting sliding seat by an angle to rotate the adjusting screw when the pulling force of the brake wire reaches a predetermined value; the yaw angle of the brake rocker arm in an initial state is increased to automatically compensate for the enlarged drum free gap.