In order to realize the high sensitivity measurement of the liquid refractive index, a sensitivity-enhanced all-fiber-optic liquid refractive index sensor based on vernier effect is proposed and demonstrated. The sensor is composed of a Fabry-Perot sensing cavity and a Fabry-Perot reference cavity in parallel. The sensing cavity is an open cavity made by offset splicing of single mode fiber, and the reference cavity is a sealed cavity, which is fabricated by fusion splicing of single mode fiber and hollow optical fiber. The free spectral ranges of the two Fabry-Perot cavities are similar, but not equal. Vernier effect is generated when the beams are superimposed via a fiber coupler to achieve the sensitization effect. Experimental results show that the refractive index sensitivity of the proposed sensor is about 9 048.78 nm/RIU, which is about 8 times that of a single sensing cavity. Due to the advantages of high sensitivity, simple structure, easy preparation and low cost, the sensor has potential application foreground in chemical and biomedical domain.