The invention discloses an ammonia injection system and a control method thereof. The ammonia injection system comprises an ammonia storage tank, an ammonia supply module, an ammonia regulating valve,an ammonia metering valve, a pressure sensor, a valve seat, an ammonia nozzle and a controller, wherein the ammonia supply module communicates with the ammonia storage tank, and the ammonia supply module is connected with the ammonia regulating valve; ammonia in the ammonia storage tank enters the ammonia metering valve, and the stable ammonia injection pressure is guaranteed by the controller bycontrolling the opening and closing of the ammonia metering valve; and the controller is combined with the ammonia injection pressure to control the opening and closing duration of the ammonia metering valve so as to inject a proper amount of ammonia. According to the ammonia injection system, double valves including the ammonia regulating valve and the ammonia metering valve are controlled by anelectronic control unit (ECU), so that the stable ammonia injection pressure can be guaranteed, thereby realizing the accurate measurement of ammonia and satisfying the CHINA VI emission requirements; the ammonia regulating valve is an electric control switching valve, the structure is simple, the process is mature, the cost is low, and compared with a mechanical pressure regulating valve, the ammonia regulating valve has obvious cost advantages and reliability advantages.