We report a simple refractive index (RI) sensor based on a twin-hole fiber (THF). The THF is fused with the single-mode fiber to realize sensing by the multimode interference. The mode transmission characteristics in the THF are numerically investigated. In the sensing experiment, when the surrounding RI changes, the wavelength shift of the transmission spectrum shows approximate linearity in the low RI region and nonlinearity in the high RI region. When a THF with the length of 10 mm is used, the highest sensitivity can be up to 1351  nm  /  RIU in the RI range from 1.430 to 1.445. Moreover, the effect of the temperature on the transmission spectrum relative to the RI is much smaller when the external temperature changes from 30°C to 80°C.