Ammonia (NH₃) gas sensor is an indispensable element in human daily life, especially in the respiratory analysis, it can perform non-invasive diagnosis of kidney disorder and hepatitis. Addressing the issue of high working temperature and achieving repeatable and stable gas-sensing properties under ambient humidity is favorable for the widespread development of NH₃sensor. In this work, a planar room-temperature NH₃sensor is manufactured through spin-coated 7,10-bis(4-(diphenylamino)phenyl)-2,3-dicyanopyrazino phenanthrene (TPA-DCPP) organic semiconductor material onto Al2O3 substrate with Au interdigital electrodes. The sensor exhibits favorable sensing response of 76.4 % and ppb-level detection ability under 98 % relative humidity (RH), also exhibits good repeatability, selectivity, and 396 days of long-term stability to NH₃. The sum frequency generation (SFG)/density functional theory (DFT) calculation results further demonstrate that the excellent sensing characteristics are due to the enhanced NH₃absorption to TPA-DCPP through the synergy interaction between NH3 and H2O under high RH. This study is expected to serve as a reference in the further development of room-temperature gas sensors and contribute to our understanding of the sensing mechanism.