A fixed-chain type composite rocker arm braking device comprises a rocker arm, a brake driving mechanism, a connecting mechanism as well as a cam of an engine, wherein the rocker arm is arranged between the cam and an air valve; the connecting mechanism is arranged between the rocker arm and the air valve; relative to the roker arm, the connecting mechanism is provided with a first position and a second position; the brake driving mechanism comprises a non-operation position and an operation position; in the operation position, the brake driving mechanism drives the connecting mechanism to reach the second position and form rigid mechanical linkage between the connecting mechanism and the rocker arm; the motion of the cam is transmitted to the air valve by the rigid mechanical linkage, so as to generate an engine braking action; and in the non-operation position, the rigid mechanical linkage is released by the brake driving mechanism and the brake driving mechanism is separated from the normal operation of the engine. According to the invention, the engine brake driving mechanism is integrated in the rocker arm or the connecting mechanism, the weight and the height of the engine are reduced, engine braking load is born by the rigid mechanical linkage, and the braking performance of the engine is enhanced.