A novel fiber-optic hydrogen gas sensor is proposed. The side of fiber cladding is polished to "D" shape, Pd and Pd/WO3 hydrogen-sensitive films are coated on the polished surface by magnetron sputtering, then an evanescent field fiber-optic hydrogen sensor is fabricated. The results show that a sensor which has composite Pd/WO3 thin film has good linerarity and repeatability. Meanwhile, compared with multi-mode fiber, single-mode fiber is more reliable.