In order to meet the requirements of high reliability places for engine speed overspeed protection, this paper proposes an engine overspeed protection system, which includes speed sensor, speed relay, three-out-of-two module and protection execution module. Among them, the speed relay, as the core of the protection, mainly includes speed signal sampling conditioning module, speed action value setting module, speed comparison module and overspeed isolation output module. The speed relay is implemented in a pure way, and the overspeed early warning protection of the engine is realized without the control of MCU and its software program, which completely solves the problems of safety, and real-time in the operation of the engine.