A new refractive index fiber sensor with simple operation, low cost and high sensitivity has been explored. The new refractive index fiber sensor is based on the principle of return strength modulation. A laser diode module and two optical power meters are used as the light source and photo-detectors, respectively. The relationship between relative return strength and refractive index is derived from the mechanism that the return loss from the end reflection interface of an optical fiber varies with the refractive index of the material contacted with the fiber. The relative return strength technology can improve measuring accuracy effectively and eliminate the measuring error resulted from the instability and internal loss of the instrument, as well as environmental effect. The sensitivity analysis of the sensor shows that the resolution of refractive index is 2×10-4 index-unit, with measurement precision of 0.01 dBm. It is found that the experimental data agree well with the theory values, and the relative error is smaller than 5% within the refractive index ranging from 1.30 to 1.45.