In this paper, WO₃-Pd₂Pt-Pt nanocomposite films were deposited on a single mode fiber as the hydrogen sensing material, which changes its reflectivity under different hydrogen concentration. The reflectivity variation was probed and converted to an electric signal by a pair of balanced InGaAs photoelectric detectors. In addition, the performance of the WO₃-Pd₂Pt-Pt composite film was investigated under different optical powers, and the irrigating power was optimized at 5 mW. With the irrigation of this optical power, the hydrogen sensitive film exhibits quick response toward 100 ppm hydrogen in air atmosphere at a room temperature of 25 °C. The experimental results demonstrate a high resolution at 5 parts per million (ppm) within a wide range from 100 to 5000 ppm in air. This simple and compact sensing system can detect hydrogen concentrations far below the explosion limit and provide early alert for hydrogen leakage, showing great potential in hydrogen-related applications.