Abstract: Real-time monitoring of characteristic dissolved gases in oil-immersed transformers is crucial for early fault diagnosis in power systems. This paper, based on first-principles density functional theory, designs a high-performance gas sensor using a single-atom Ir/Rh modified Janus WSeTe monolayer. Results show that, driven by the Janus polarization electric field, the single atoms are stably anchored to the Te surface. IrTeW and RhTeW overcome the intrinsic surface inertia of the materials, exhibiting strong chemisorption with all four characteristic gases. The high sensitivity stems from the strong orbital hybridization between the p orbitals of gas molecules and the d orbitals of noble metals, as well as the classic "π-feedback" mechanism, a process that breaks the local electronic states of the system. Based on performance evaluation, an innovative "dual-modal" engineering application strategy for this system is proposed. This research provides a novel material design paradigm and microscopic mechanism support for developing next-generation high-performance transformer fault gas sensors.