The utility model discloses an automatic adjusting arm of an automobile brake. The arm comprises a shell, a worm wheel, a worm, a cover plate, a control unit, a torsional spring, a retaining spring, a spring seat, and a small end cover, wherein the worm wheel and the worm are installed in a mounting cavity corresponding to the shell; the worm wheel is meshed and connected with the worm; the control unit comprises a gear, a rack, and a shifting fork; the rack is connected together with the gear in the shell through a through hole of the shell; the gear is connected together with the worm through the torsional spring; one end of the worm is fixed on the shell through the small end cover and a check ring; and the other end of the worm is connected together with the spring seat through the retaining spring and fixed on the shell. The automatic adjusting arm does not occupy unnecessary space, and is more compact in arrangement and structure. Due to fixing of the cover plate and the small end cover, the arm is very convenient in installation and disassembly, and the one-way clutching mechanism consisting of the gear, the worm wheel, the torsional spring, the retaining spring and the spring seat leads to a better effect of clearance adjustment.