The tungsten trioxide sol is prepared using the tungsten powder-hydrogen peroxide-polytungstic acid method, doped using chloroplatinic acid, and spin-coated into a platinum-doped nanotungsten trioxide film exhibiting hydrogen-induced discoloration. The film properties are analyzed using X-ray diffraction, field-emission scanning electron microscopy, and energy-dispersive spectroscopy. The film is amorphous and exhibits a flat, bump- and crack-free surface over which platinum metal and loose porous-structured tungsten trioxide particles are evenly distributed. A transmission optical-fiber hydrogen sensor system is fabricated using the prepared film. When hydrogen molecules are present, the film chemically reacts through platinum catalysis, thereby changing the film refractive index. The hydrogen concentration is determined by measuring the transmitted light intensity. The hydrogen-sensitive film spin-coated for 60 s using a homogenizer rotating at 3100 r/min and subsequently annealed at 400 ℃ for 60 min exhibits a hydrogen-passing response time of approximately 62 s at a H₂ concentration of 5%. The average change rate of the signal response amplitude is 91.54% before and after hydrogen sensing, and the film exhibits good repeatability and stability.