Microwave gas sensor (MGS), being as a kind of new-developed sensing device, possesses the ability to resolve the poor stability, imprecise selectivity, slow response speed and other problems plaguing room-temperature gas sensors. However, the nature of gas sensing mechanism of MGS has not yet been revealed, and the sensitive materials currently applied on MGS are relatively unstable and insensitive under high humidity. Here we have for the first time reported a highly sensitive, selective, and stable MGS composed of ultra-narrowband microwave filter transducer and organic-inorganic hybrid sensitive material (multilayered structured polyaniline (PANI)SnO2). Owing to the standout capacity of ammonia (NH3) adsorption, the high resistance to water poisoning, and the efficient tuning of relative permittivity ascribed to p-n (PANI-SnO2) heterojunction, the organic-inorganic hybrid based MGS operating at room-temperature can selectively detect low concentration of NH3 with high sensitivity, fast response speed, and anti-humidity capacity, which has never been reported before. This work develops a new class of microwave sensing functional materials, and demonstrates that the concept in tuning relative permittivity for constructing outstanding performance microwave gas sensor.