The invention discloses a brake positioning control method for an engine. When an engine needs to normally work, one end of a braking rocker arm is separated from a braking cam, and the other end of the braking rocker arm is separated from a braking pad. When the engine needs to be braked, gaps between the breaking rocker arm and the braking cam and between the breaking rocker arm and the braking pad can be eliminated. The invention also discloses a positioning mechanism for the engine brake. The a positioning mechanism is composed of a braking cam, a braking rocker arm, a rocker arm shaft, a braking press cake, a vent valve and a braking rocker arm positioning mechanism, wherein the braking rocker arm is installed on the rocker arm shaft in a swinging way; one end of the braking rocker arm approaches to the braking cam, and the other end of the braking rocker arm approaches to the braking press cake; and the braking press cake is a braking piston, or the elephant foot pat of an engine, a valve bridge and a rocker arm. The positioning mechanism is integrated in the vent valve system of the engine to prevent the braking rocker arm, the braking cam, the braking press cake and other engine components from being suddenly collided, thereby realizing stably switch between the braking state and normal working state of the engine. The engine with the braking rocker arm positioning structure occupies small area and is convenient to install and carry.