As a typical representative of conductive polymers (CPs), poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS) is intensively employed for chemiresistive ammonia (NH 3 ) sensing on account of its favorable aqueous solubility, benign environmental stability, and outstanding room-temperature conductivity; however, it is severely plagued by low sensitivity and sluggish reaction kinetics. To circumvent these limitations, the guest-alkalized cellulose nanofibers (AC) were introduced into the host PEDOT:PSS matrix by the layer-by-layer spraying assembly method (LBLSA) in this work. The componential proportion-optimized PEDOT:PSS/AC/PEDOT:PSS (P/AC/P) sensor delivered a large sensitivity of 20.2%/ppm within 0.1-3 ppm of NH 3 at 21 °C@26% RH, an experimental limit of detection (LoD) as low as 30 ppb, a high response of 18.1%, and a short response/recovery times (4.8/4.0 s) toward 1 ppm of NH 3 , which ranked among the best cases thus far. Also, excellent repeatability and long-term stability and selectivity were demonstrated. Meanwhile, the flexible P/AC/P sensors worked well under various bending angles and bending times. This work combines a green material system and a facile film deposition method to overcome the liquid dispersion incompatibility when preparing a multicomponent mixture for swift trace NH 3 detection. The universality and extensibility of this methodology endow a broad prospect in the field of future wearable optoelectronic systems.