Explosive developments in modern society bring huge fire loads. Previous fire detections at early stages are basically enabled by recognizing abnormal high-temperatures, smoke particles, and flame light signals. However, the identification of these characteristic signals is generally accompanied by an open flame or smoke, which makes it difficult to prevent further serious damage. Herein, a latent-firedetecting strategy of trace ammonia (NH 3 ) analysis based on nanohybrid Ti 3 C 2 T x MXene/MoS 2 is proposed. Benefiting from nanoscale high-density Schottky heterojunctions between MoS 2 and Ti 3 C 2 T x MXene, ultrafast (3 s @100 ppm), sub-ppm (200 ppb minimum), and high-sensitivity (81.7% @100 ppm and 10.2% @200 ppb) detection of NH 3 are enabled. An assembled latent-fire-detecting olfactory system (LFOS) based on MXene/MoS 2 and interdigital electrodes can monitor trace NH 3 releases from different materials (wool, leather, foam, and nylon) during thermal decomposition at latent stages. Notably, the LFOS can detect fire threats at least 84 s earlier than commercialized smoke detectors, providing more fire dealing time and an escape period; this offers a promising latent-fire-warning approach for eliminating fire treats at an early stage.