This study proposes a Brillouin refractive index sensor based on the evanescent field effect of a micro-nano fiber. Multipeak Brillouin spectrum caused by hybrid acoustic field of the fiber is used for liquid refractive index sensing. Experimental results show that, for a 2 μm-micro-nano fiber, Brillouin peaks at approximately 10.4 GHz have a relatively high signal-to-noise ratio in NaCl solution, and their frequency shifts have a linear dependence on the liquid refractive index. A refractive index sensing sensitivity of 369.6 MHz/RIU (RIU is refractivity unit) is obtained within the range of 1.3333-1.3612. The proposed sensor will provide a new alternative for refractive index sensing.