A silica-core Bragg fiber surface-plasmon resonance (SPR) sensor with high design flexibility has been developed for refractive index measurement. In this geometry, an HE11-like Bragg fiber core mode is designed to excite the surface plasmonic wave. It is shown that reduction of core size enables measurement of refractive index significantly lower than that of the silica core. This makes the device useful for biosensing or chemical sensing applications handling aqueous samples. The proposed sensor is fabricated using a unique rf-sputtering method, and SPR-induced extinction of more than 20 dB is observed.