The invention discloses a speed control protecting device for a diesel engine during emergency braking of a locomotive. The speed control protecting device comprises a driver controller and a diesel engine speed control module (SCM). A normally-closed contact K1 of an emergency stop control relay (ESCR) and a normally open contact of a main alternator field contactor (AFC) are connected in parallel and then connected between the driver controller and the diesel engine SCM. The ESCR is connected with the main AFC in parallel, connected with a mechanical control switch S2 in series and then connected to the positive pole and the negative pole of a power supply. By the adoption of the protecting device, after the locomotive is braked emergently and the diesel engine is unloaded suddenly, the rotating speed of the diesel engine can be automatically decreased to the idling rotating speed. The situation that the diesel engine of the locomotive gallops since the rotating speed is out of control can be avoided, the locomotive damage accident is prevented, and the operation safety of the locomotive is improved. Meanwhile, the oil consumption of the locomotive is reduced, and the economical efficiency of the locomotive is improved.