An optical fiber sensor based on a second-order superstructure fiber Bragg grating (SFBG) is experimentally investigated. In general, a first-order SFBG has been available for many years and has been widely applied in optical fiber communication systems and fiber sensing systems. An SFBG is composed of a fiber Bragg grating (FBG) and a long-period grating (LPG), so it can be regarded as a periodically modulated FBG. In this study, to the best of our knowledge, a second-order SFBG is successfully fabricated for the first time. Moreover, we are the first to experimentally demonstrate that the refractive index sensitivities of a second-order SFBG sensor are larger than those of the first-order SFBG. This configuration and technique can provide simplestructure, low-cost, and high-sensitivity innovative devices for application to refractive index measurement systems.