In order to avoid the temperature cross-sensitivity during refractive index measurement, a sensor based on a superimposed long period gratings fabricated in few mode fiber was proposed. The superimposed long period grating was inscribed under the CO2 laser irradiation as following: first, a long period fiber grating with a period of 654 μm and length of 30 mm was inscribed, then the few mode fiber was rotated 180° to inscribe second long period fiber grating with a period of 819 μm and the same length. Two resonance dips at 1 487.2 nm and 1 533.0 nm appeared in the transmission spectrum, which were formed by coupling between different guided modes. The difference of two resonance dips increases with the refractive index while is constant with temperature increase, which can eliminate temperature cross-sensitivity during refractive index measurement. The experimental results show the temperature sensitivity of the sensor is -0.002 5 nm/℃ in 20~70 ℃ temperature range and the refractive index sensitivity of 143 nm/RIU in 1.333 3~1.376 6 refractive index range. The temperature sensitivity of the sensor is much lower than the refractive index sensitivity, which means that it is insensitive to temperature. Compared with traditional optical fiber sensors, the sensor has the advantages of small temperature cross-sensitivity, high refractive index sensitivity, small size and compact structure, and it can be widely used in industry, water conservancy, medicine and other fields.