F-P optical resonant cavity （Fabry-Pérot cavity） is widely used for sensing refractive index, temperature,pressure, etc. The closed fiber F-P resonant cavity limits the types of substances to be measured and their measurement range due to the long resonant cavity. Herein, a short cavity length open F-P refractive index sensor was designed and fabricated based on thin film technology and lithography , which definitely increases the measurement range and high sensitivity,and is suitable for gases and liquids. Theoretical research was conducted on the influence of mirror reflectivity and cavity length on the sensitive parameters of open F-P cavity, and an open F-P resonant cavity structure and characteristics were all determined, including upper and lower high reflectivity mirrors （99.7%） and short cavity length（3.305 μm）. The FDTD simulation results showed that its free spectral range was as high as 42nm, the sensitivity was as high as 385.85nm per Refractive Index Unit（RIU）, and the refractive index measurement range was one order of magnitude higher than traditional F-P cavities, reaching to 0.129RIU. In addition, a refractive index testing and verification system was built, and refractive index testing experiments were all carried out. Experimental verification showed that the greater the change in refractive index, the greater the shift in the center wavelength. The average change in the center wavelength was 1.79 nm/RIU, and the experimental sensitivity was determined to 346.65 nm/RIU. The results lay a theoretical and practical foundation for low-cost,wide measurement range, and high-precision liquid/gas refractive index sensors.