Refractive index is an important parameter of materials. In many fields such as chemical analysis, environmental protection and medical health, the detection and measurement of refractive index have been widely used. Among them, bending or called U-shaped optical fiber is the simplest structure to achieve refractive index sensing, using the basic principle that the U-shaped bending loss of optical fiber can be affected by the external environment of bending optical fiber, combined with the advantages of ordinary single-mode fiber, which has high flexibility and hard to break, etc. A high sensing sensitivity, simple structure and low cost distributed refractive index sensing scheme based on Rayleigh scattering spectrum detection in U-shaped fiber is proposed. When the bending diameter is 0.7 cm, the sensing sensitivity reaches the maximum value of 42.57 nm/RIU in the refractive index measurement range of 1.333 0~1.377 3, and presents a good linear sensing response, which makes the application and popularization of disposable cheap refractive index sensing probes such as medical devices possible.