We report in this paper a method of measuring the cavity spacing of a Fabry-Perot interferometer with error less than several tenth of wavelength. First the central fractional order was obtained from interference pattern by a OMA system, which consists of a diode array and a microcomputer, for 32 different wavelengths and He-Ne 6328 A line. Then by using the method of excess fractions to process data, the absolute interference order and cavity spacing of F-P interferometer was worked out from the data corresponding at least to 4 different wavelengths. Further more, the relative reflection phase dispersion curve for silver mirror was obtained by using the data fitting method.