A highly sensitive hydrogen sensor coated with Pt/WO3 powder with an Erbium-doped fibre amplifier (EDFA) is proposed and experimentally demonstrated. The sensing head is constructed by splicing a short section of tapered small diameter coreless fiber (TSDCF diameter of 62.5 μm, and tapered to 14.5 μm) between two single-mode fibres. The Pt/WO3 powder adheres to the surface of PDMS film coated on the TSDCF structure, which is sensitive to hydrogen. An EDFA is introduced into the sensor system to improve the quality factor of the output spectrum and thus improve the sensor’s resolution. As the hydrogen concentration varies from 0% to 1.44%, the measured maximum light intensity variation and the sensor’s sensitivity are -32.41 dB and -21.25 dB/%, respectively. The sensor demonstrates good stability with the light intensity fluctuation of < 1.26 dB over a 30-minute duration.