In SF6 insulated high-voltage gas power systems, H2O is the most problematic impurity which not only decreases insulation performance but also creates an acidic atmosphere that promotes corrosion. Corrosion damages electrical equipment and leads to leaks, which pose serious safety hazards to people and the environment. A QEPAS-based sensor system for the sub-ppm level H2O detection in SF6 buffer gas was developed by use of a near-infrared commercial DFB diode laser. Since the specific physical constants of SF6 are strongly different from that of N2 or air, the resonant frequency and Q-factor of the bare quartz tuning fork (QTF) had changed to 32,763 Hz and 4173, respectively. The optimal vertical detection position was 1.2 mm far from the QTF opening. After the experimental optimization of acoustic micro-resonator (AmR) parameters, gas pressures, and modulation depths, a detection limit of 0.49 ppm was achieved for an averaging time of 1 s, which provided a powerful prevention tool for the safety monitoring in power systems.