In this work, we employed straightforward all-in-one synthesis technology to fabricate room temperature flexible ammonia sensors that integrate sensitive materials and flexible substrates. The sensitive sensor material was composed of a nontoxic and environmentally friendly cellulose-based material and a biomass acid, which was selected to ensure environmental safety during exhaled gas detection. The optimized sensor displayed excellent selectivity for NH   3 over a range of interfering gas species, achieving a significant NH   3 response (Za/Zg = 5.30) for 20 ppm NH   3, along with a low actual detection limit of 300 ppb at room temperature under high humidity conditions (80 % RH). As flexible devices, these sensors were capable of working under deformation conditions and have non-degenerated sensing performance. The humidity-activated NH   3 sensing mechanism was extensively examined using complex impedance spectroscopy (CIS) and quartz crystal microbalance (QCM) techniques. Simultaneously, experiments involving the detection of simulated exhaled gases were conducted to investigate the potential of the sensor in this field.