A kind of optical fiber refractive index sensor based on a fiber core etched air-bubble in-fiber Mach-Zehnder interferometer was proposed and demonstrated. A core etched standard single mode fiber was spliced to a thinner core single mode fiber to form an air bubble at the connecting point. By this method, a section of thinner core fiber was cascaded between two air bubbles to form a structure of (Air-Bubble)-Thin core fiber-(Air-Bubble) in-fiber Mach-Zehnder interferometer as a refractive index sensor. The air-bubble in the sensor serves as an optical coupler for modes conversion and the thin core fiber serves as sensing beams. The transmission spectrum of sensor was studied by experiment. The results show that, the dip power of the interference fringes changes with respect to surrounding refractive index with a good linearity,at the same time,the dip wavelength keeps low dependence on RI. The sensitivity of the sensor is －216.21dB/RIU in the range of 1.345~1.389. The sensor is a good potential candidate for bio-chemical measurements.