A LOOP-type micro-nano fiber resonant ring is fabricated. The factors influencing the shape and characteristics of the resonant ring are analyzed, and it is applied to the refractive index sensing experiment. Results show that the free spectral range of the micro-nano fiber resonant ring is inversely proportional to the ring size. With increasing of the external refractive index, spectrum of the micro-nano fiber resonant ring shows a tendency to shift to long wavelength. That is to say, the red shift phenomenon occurs. There is a linear relationship between the red shift of the wavelength and the refractive index, and the refractive index sensitivity of 1427.86 nm/RIU is achieved. The designed refractive index sensing structure based on micro-nano fiber resonant ring is simple, integrated and sensitive.