The invention relates to an ammonia escape prevention device in desulfurization and denitrification of flue gas by using an ammonia process. The ammonia escape prevention device comprises an oxidized air and original flue gas inlet, a purified flue gas outlet, a desulfurization tower, a mist eliminator, an ammonia washing section, an absorption section, a concentration section, an oxidation section, a circulating ammonia washing pump and a circulating ammonia washing tank, wherein the concentration section is connected upwards to the absorption section, the absorption section is connected upwards to the ammonia washing section, the ammonia washing section is connected upwards to the mist eliminator, the mist eliminator is connected upwards to the desulfurization tower, the purified flue gas outlet is connected and formed in the desulfurization tower, the ammonia washing section is connected to the circulating ammonia washing tank through a pipeline, the circulating ammonia washing pump is arranged on the pipeline which is connected from the ammonia washing section to the circulating ammonia washing tank, the oxidation section is connected to the absorption section and the concentration section through pipelines, a primary circulating pump is arranged on the pipelines which is connected from the oxidation section to the absorption section and the concentration section, the ammonia washing section is connected to the circulating ammonia washing tank through the pipeline and meanwhile connected to the absorption section and the oxidation section through the pipelines, and one ammonia washing section is arranged in front of the mist eliminator. The ammonia escape prevention device is simple in overall structure, convenient to operate and use, good in stability and high in reliability.