A Pd/Ag metal-coated optical fiber sensor with high sensitivity to hydrogen concentrations change which is achieved by exciting surface plasmon resonance (SPR) is presented. Simulation results show that with the increase of the refractive index of sensing area, effective refractive index of the SPR mode of metal-coated side-polished "D" shape fiber increases; while with the increase of the thickness of metal coating, the effective refractive index of the SPR decreases. Experimental results show that when the hydrogen concentration is 4% , transmission power changes as high as 130 nW. The fiber optic SPR sensor improve the stability and sensitivity of the the hydrogen sensor. This sensor would be of great interest to produce novel and enhanced devices for chemical and biological sensing.