With the rapid development of the Internet of Things (IoT), the smart agriculture has played a key role in modern society. For the purpose of constructing smart agriculture systems, the distributed sensors should be applied, especially for the ammonia gas (NH3) sensing devices with versatile functions. However, the traditional NH3 sensors with the resistive heater severely restrict the development of room-temperature gas sensors, flexible electronics and integration of circuits in the practical application of IoT. Herein, a light-driven NH3 sensor based on plasmonic-functionalized metal carbides/nitrides (MXenes) is developed to achieve the room-temperature, ultra-sensitive NH3 sensing towards smart agriculture. The in-situ oxidized niobium carbide (HT-Nb2CTx) serves as NH3 adsorption sites as well as conductive layers on flexible PET substrates to induce a large charge transfer. Density functional theory (DFT) calculations further verify that NH3 adsorption energy is remarkably increased on in-situ oxidized sites. Most noteworthy is that plasmonic Au nanorods, as the local heating sites of 20 nm replace the extra resistive heater to promote the NH3 sensing reaction under 980 nm light, taking the advantage of photo-thermal effects of localized surface plasmon resonance (LSPR). Such Au/HT-Nb2CTx based gas sensors exhibits the trace NH3 sensing ability with a low limit of detection (LOD=500 ppb) and full recovery. The sensitivity (80 % for 100 ppm of NH3) is more than two times higher compared with the counterparts without Au NRs. A light-driven, portable NH3 sensing and alarming system consisted with the gas/temperature/humidity sensors is wirelessly connected to a mobile phone to realize the “round-the-clock” environment monitoring. This proof of the sensing device using Au/HT-Nb2CTx exhibits the potential application in the next-generation NH3 monitoring system in the future smart agriculture.