The design and application of a sensing device, based upon Fabry-Perot interferometer, for measuring refractive index of aqueous solution or concentration are presented. It is manifested by a successful measurement of the refractive index of ethanol solution, with fairly high precision, by utilizing a single-mode fiber for laser transmission and a two-unit CCD for image acquisition. Theoretically, the measurement principle and the feasibility of the device are analyzed; and the experimental devices are constructed with a He-Ne laser, a segment of single-mode fiber, an F-P cavity and a CCD head. Through counting the number of the concentric interference rings, with calculation the refractive index and the solution concentration are retrieved with high precision. The dynamic evolution of these rings is demonstrated, corroborating the measurement process. A refractive index change of 10-5 and a corresponding concentration variation of about 0.04% are inferred from the measured image data.