Abstract: Mastering the distribution characteristics of escaped ammonia after ultra-low NO_x emissions, especially under the high denitration efficiency of SCR, is of great significance for clarifying the characteristics of ammonia emission sources in coal-fired units and improving the reliability of a boiler's safe operation. In this study, a typical ultra-low emission unit was used as the test object. Under 320MW load and 96.6% denitration efficiency, the ammonia of all emission channels was sampled and analyzed at the same time to obtain the distribution characteristics of escaped ammonia. Ammonia mass balance was calculated to verify the accuracy of the test results. The results show that the ammonia concentration at the outlet of SCR was 0.60mg/m³, which meets the requirements in HJ562-2010. With the in-depth treatment of flue gas, the ammonia concentration is synergistically removed to a lower level. The ammonia concentration emitted from the chimney is only 0.06mg/m³. Except that 11.7% of the escaped ammonia reacts with SO₃ to form NH₄HSO₄ attaching in the air preheater, the mass flow ratios of ammonia in the four emission channels of fly ash, gypsum, desulfurization waste-water and exhaust flue gas are respectively 25.6%, 42.8%, 17.9% and 13.7%.