A technique for accurate measurement of the refractive indexes of liquid and gas was proposed. The approach, based on Fabry Perot (F P) interferometer and using the central wavelength of transmission spectrum to elicit the measurand, essentially eliminates the errors aroused from light source fluctuation. Such a system was developed. The optical signal was transmitted through fibers, and a tunable optical filter is used to collect the sensing data. Using the test data of refractive indexes of a set of alcohol solution with different concentrations, the analytical relation between refractive index and central wavelength of transmission spectrum was educed with least squares linear regression method, the regression error is less than 0 00005. Refractive indexes of distilled water and acetone were measured respectively; the measuring error is no more than 0 0001.