The utility model relates to a vehicle electronic mechanical brake system clearance automatic adjusting device which includes an output shaft; the internal hole of the output shaft is fixedly connected with a rotary shaft in the circumference direction; the output shaft is sleeved with a self-adjusting gear which can be relatively rotated in the angle range Alpha along the circumference of the output shaft; the angle alpha is restricted by the position limit device arranged on the output shaft and the self-adjusting gear; the self-adjusting gear is provided with a hackle gear which is counterclockwise oblique; an oblique wedge-typed head of a floating unidirectional position limit pin arranged on the shell is inserted between a plurality of gears; the gear surface D with large oblique angle of the head is butted with the gear surface B of the gear; the C surface with small oblique angle of the head is butted with the gear surface A of the gear, constituting the reserve locking of the self-adjusting gear. The utility model has the advantages of producing the clearance self-adjusting function when executing the vehicle brake action, having high invariance of the vehicle brake response speed, small space restriction, good smartness, simple and reasonable structure, easy manufacture and maintenance, low cost of manufacture and maintenance.