An ultra-high sensitivity refractive index sensor based on double biased fiber Mach-Zehnder interferometer (MZI) is proposed.The interference mechanism of biased MZI is analyzed theoretically.The refractive index sensing characteristics of the multimodal biased fiber,the variation of refractive index sensitivity in different biased lengths and refractive index ranges are simulated and analyzed by beam propagation method (BPM),and the sensing characteristics of single-biased and double-biased are comparedThe simulation results show that the refractive index sensitivity of single-biased MZI is -5 557 nm/RIU when the refractive index range is 1.333 0—1.334 0,and the refractive index sensitivity of double-biased MZI is -14 071 nm/RIU,which provides a import theoretical basis for high sensitivity measurement of liquid refractive index.The sensor has a compact structure and the overall length is only 1 200 μm,which has important application value in the field of liquid refractive index measurement.