A fiber sensor based on the single mode-multimode-single mode (SMS) structure for refractive index (RI) sensing is proposed. A kind of no core fiber is used as the multimode waveguide. Compared with the conventional SMS with an etched multimode fiber, the proposed sensor is easy to design and fabricate. A model RI sensor based on the proposed structure is designed and experimentally tested with sucrose solutions with different RI values. The results show that the sensitivity of the sensor is 431.4 nm/RIU on average in the range of 1.356~1.392. The experimental results agree well with the simulation results. Besides, computer simulation is also performed to systematically analyze the sensor′s characteristics. The results show that the sensitivity of the sensor can be improved by reducing the diameter of the no core fiber.