The utility model discloses an ammonia detecting device used for flue gas pipeline denitration. The ammonia detecting device comprises a first hose, the gas inlet end of the first hose is connected with a flue gas pipeline through a sampling gun, the gas outlet end of the first hose is connected with a heat exchanger, a fan heater is connected with the right side of the heat exchanger, an air filter core is connected with the right side of the fan heater and connected with a first absorbing bottle through a second hose, a temperature sensor is arranged in the second hose and connected with the fan heater through a control system, the gas outlet end of the first absorbing bottle is connected with a second absorbing bottle, and the gas outlet end of the second absorbing bottle is connected with a gas extracting pump. A heat preservation layer can reduce heat loss of flue gas samples; the heat exchanger uses heat of own flue gas for heating the flue gas samples to achieve energy conservation and emission reduction; the fan heater can ensure that the flue gas samples are maintained at the optimal temperature, so that the flue gas samples are completely absorbed by absorbing liquid; the air filter core can filter the flue gas samples secondarily; the absorbing liquid in the two absorbing bottles can flow into a collecting bottle by opening a valve, and operation is safe and convenient.