The utility model provides an on-line ammonia nitrogen monitoring system. The system comprises a detection device, a heating device, a feeding device and a data processing device, wherein the detection device is used for detecting ammonia nitrogen concentration signals and temperature signals of liquids; the heating device is used for sending a heated liquid to a detection flow path through a pipeline; the feeding device comprises a peristaltic pump, a pipeline combination unit, a sample overflow device, guide sample containers, a cleaning solution container and a reagent container, the peristaltic pump sends pumped liquids to the pipeline combination unit through a pipeline, and the pipeline combination unit converges the input liquids and then sends the liquids to a heating flow path through a pipeline; and the data processing device is electrically connected with an ammonia sensor, a temperature sensor, an electric constant temperature source, the peristaltic pump, an electrically-controlled valve and the sample overflow device, and is used for controlling the operations of the electric constant temperature source, the peristaltic pump, the electrically-controlled valve and the sample overflow device, acquiring and processing the ammonia nitrogen concentration signals and the temperature signals, and generating and outputting monitoring results. The utility model can realize an accurate and timely on-line monitoring of the ammonia nitrogen contents in reclaimed water and circulating cooling water of a thermal power plant.