The development of hydrogen sensors with fast response and reliable stability is the key challenge for hydrogen leak detection. Fiber-optic sensors are ideal for hydrogen sensors due to their compactness and inherent safety, but their response speed and reliable stability still need to be improved. Herein, we proposed and experimentally developed a tilted fiber Bragg grating (TFBG) fiber-optic hydrogen sensor functionalized with urchin-like $\mathrm { W_{18}O_{49}}$ nanospheres. The $\mathrm { W_{18}O_{49}}$ nanospheres have a large number of tiny spines with gas channels and active sites for capturing hydrogen molecules to enhance the reaction between the TFBG’s evanescent wave and the hydrogen molecules. Experimental results demonstrated that such sensor has an ultrafast ( $4 \sim 5$ s), highly reproducible response (99.8%) for repeat hydrogen detections with concentration from 0% to 3.0% at room temperature.