Hydrogen is explosive and hydrogen sensors are used in hydrogen monitoring work. The hydrogen sensor films used in previous hydrogen monitoring work were WO₃-based and Mg-based hydrogen sensor films, which only available in an aerobic environment. Hydrogen sensing films for monitoring hydrogen concentration in an oxygen-free environment remain to be further investigated. Tantalum is stable in nature and has a high solubility for hydrogen in oxygen-free environment. In this paper, 40 nm Ta_0.88Pd_0.12~10 nm Pd~6 nm Pt~40 nm PTFE multilayer films were deposited on the end face of single mode optical fiber for hydrogen concentration monitoring for the absence of oxygen. The reflectivity of the deposited film under different hydrogen concentration was probed by the sensing demodulator. The sensing performance were investigated by a series of hydrogen sensing experiments. Firstly, the sensing film are designed for hydrogen sensing in oxygen-free environment. The Ta_0.88Pd_0.12 thin film is used as basal layer for sensing. Palladium film can improve the selectivity of hydrogen sensing film. Tantalum and palladium absorb hydrogen and become TaH_x and PdH_x. This phenomenon will result in a decrease in the reflectivity of the film, so that hydrogen concentration can be monitored by the change of reflected light intensity. Platinum film has good catalytic effect and excellent oxidation resistance, so it is employed as a protective layer. PTFE is hydrophobic and can hinder the adsorption of water molecules on the surface of the hydrogen sensing film. Moreover, it has good stability under various ambient environment, which can reduce the negative influence of temperature and humidity. The hydrogen sensing probe was fabricated by magnetron sputtering aforementioned multilayer films. The microscopic morphology of hydrogen sensing film was characterized by scanning electron microscope. Elements of hydrogen sensing thin film were analyzed by energy dispersive spectrometer. The phases of hydrogen sensing film were analyzed by X-ray diffractometer. Secondly, a fiber optic hydrogen sensing system based on Ta-based hydrogen sensing film was constructed, including amplified spontaneous emission light source, attenuator, coupler, spectral acquisition module, reference fiber grating and the fabricated sensing probe. The spectral response of the reference fiber grating with high-reflection was acquired by a compact spectral acquisition module with the range of 1 520~1 570 nm. The Reflection peak intensity (I₁) and background intensity (I₂) were obtained simultaneously. Reflection peak intensity (I₁) of the high-reflection fiber grating is hardly affected by the reflectivity of hydrogen sensing film and is used as the reference signal. The ratio of I₁ over I₂ is traced as main measuring parameter to enhance the signal noise ratio of sensing system and to suppress the other noise induced by light source fluctuations, insertion loss, and fiber bending. Finally, we investigated the hydrogen sensing performance of the fabricated sensing probe. The probes are characterized in different hydrogen concentration provided by a gas mixer including two gas flow meters with N₂ as carrier gas. A series of experiments are carried out to verify the sensitivity and repeatability of the fiber optic hydrogen sensing system with the proposed Ta-based probe. Three on/off cycles under a hydrogen concentration of 3 000 ppm are conducted. When the sensor is put in nitrogen, the value of I₁/I₂ is on a lower level. When the hydrogen with a concentration of 3000 ppm is turned on, the value of I₁/I₂ rises to a higher value each time. The results have shown the sensor has a good repeatability and recovery during hydrogen on/off cycles. Multiple experiments under gradient hydrogen concentration with a lower range of 100 ppm~1 000 ppm and a higher range of 1 000 ppm~20 000 ppm show that the different hydrogen sensitivity for different hydrogen concentration ranges. When the hydrogen concentration is in the range of 100 ppm~1 000 ppm, the sensitivity of sensor probe is the largest. The theoretical resolution is 20 ppm in the range of 100 ppm~1 000 ppm hydrogen concentration. This is because the hydrogen sensing film can easily reach saturation in the absorption of hydrogen at high concentrations of hydrogen. As the hydrogen concentration increases over 1 000 ppm, the reaction rate of the sensing film with hydrogen becomes slower. The result implies the sensor probe presents better sensitivity towards lower hydrogen concentration. In conclusion, the sensor probe proposed in this paper has the potential to monitor hydrogen concentrations in an oxygen-free environment and is suitable for monitoring the change of low concentration hydrogen gas.