This paper proposes a trace hydrogen sensing system based on distributed feedback fiber laser (DFB-FL). The sensor head was fabricated by inserting the DFB-FL into a glass capillary coated with hydrogen sensitive material. The wavelength of the DFB-FL changed under the exothermic reaction of Pt-WO3 with hydrogen and was demodulated by a Michelson interferometer. The experimental results at hydrogen concentration of 50 to 3800 ppm show that the hydrogen sensitivity of the system is 0.0386 pm/ppm and the linearity index R2 is >0.99. The repeatability of the system is 99.4% and the accuracy is 97.43% at hydrogen concentration of 50 ppm. The figure of merit of the system is 0.0385. The proposed DFB-FL sensor with good repeatability and anti-electronic interference has application prospects in many fields such as the detection of trace hydrogen in transformer oil.