In this paper, a simple fiber optical sensor system used to measure the refractive index (RI) is proposed. It only consists of one optical source, one 2 2 optical coupler with two fiber sensing ends, and one mechanical 1 2 optical switch that is controlled by the drive voltage applied on it. The operation principle is based on relative Fresnel reflective intensity. The refractive indices of chemical species including benzene, ethanol, methanol, acetone, and glycerol are measured at 1550 nm wavelength. The dependence of the RI of tap water on the temperature is also addressed. A short-time measurement precision of 8 and a long-time measurement precision of 5 were obtained. Applying the relative technique, the errors resulted from the fluctuation of light source and the influences of environment are effectively eliminated. The sensor system is of low cost, simple operation, high sensitivity, and precision.