The invention discloses an ammonia injection system suitable for a desulfurization and denitrification device adopting a carbon-based catalyst and a method. An adsorption tower is provided with an entrance flue gas box, a flue gas system and the ammonia injection system are communicated with the entrance flue gas box, and the ammonia injection system comprises an ammonia gas system, an air system and a mixed gas system for mixing ammonia gas and air, wherein the mixed gas system comprises a static mixer and a mixed gas pipeline for communicating the static mixer with the entrance flue gas box, and multiple nozzles are arranged on the mixed gas pipeline and in the position where the mixed gas pipeline is communicated with the entrance flue gas box; the ammonia gas system comprises an ammonia source, an ammonia gas pipeline connected between the ammonia source and the static mixer as well as a first flow meter and a pneumatic flow regulating valve which are arranged on the ammonia gas pipeline sequentially in the ammonia gas flowing direction; the air system comprises a centrifugal fan, an air pipeline connected between the centrifugal fan and the static mixer as well as a first motor-driven baffle valve, a second flow meter, a heater and a thermometer which are arranged on the air pipeline sequentially in the air flowing direction. The system and the method are stable and safe.