To develop a kind of easy-fabrication and stable optical fiber sensor, a new method for fabricating the fiber sensors was proposed, and the sensing performance of a U-shape bending single mode fiber was investigated. A section of SM-28 single mode fiber was bent into U-shaped structure with variety of radii of curvature, and then the U-shaped structure is fixed. After that, the U-shaped portion of the fiber was immersed in liquid with different refractive indices respectively. Meanwhile, the transmission spectrum of the U-shaped optical fiber was recorded. The modal interference theoretical analysis shows that the core guiding mode and the cladding mode, which propagate independently in the bending section, interferes each other in the straight region, and that the interference wavelengths depend on the bending radius and surrounding refractive index. Obvious interference spectra can be clearly observed on the U-shape bending fiber with radius ranging from 2.5 mm to 5.0 mm. The interference wavelengths red-shift as the surrounding refractive index increases from 1.30 to 1.43RIU. Higher sensitivity could be achieved when the bending fiber has a lager bending radius. In the case of the bending radius being 5.0 mm, the sensitivity reaches to 207nm/RIU and 1 220 nm/RIU in the SRI region of 1.30~1.40 RIU and 1.40~1.42RIU. The characteristics of the interference dip, such as full width at half maximum, contrast are determined by the ratio of core guiding mode and cladding mode. The narrowest full width at half maximum was achieved when the bending radius was 4.5 mm of U-bend single mode fiber were discussed. A low cost and easy making sensor was proposed. The figure of merit, which integrates the sensitivity and the full width at half maximum, reach the maximal value of 43.1 RIU-1 and 191.2 RIU-1 in the surrounding refractive index region of 1.30~1.40 RIU and 1.40~1.42 RIU, when the bending radius is 4.5 mm. This sensor is fabricated with a common single mode fiber with simply bending process, and shows potential in the fiber sensing application with the features of low cost and high mechanical strength.