In this paper, based on the principle of range finding of FMCW, the range finding system of the double light path FMCW is constructed and improved and the optical fiber delay line is added in the measuring optical path to expand the measuring distance to 65 m. The influence of dispersion in equal optical frequency resampling is also researched and the resampling model with dispersion and optical fiber delay line is deduced in this paper. Therefore, the dispersion correction method that uses frequency amplitude to adjust the phase compensation coefficient in large-length range finding of FMCW is proposed. The experiments show that the optical fiber delay line is added within the range of 45-65 m and the dispersion is corrected effectively. After dispersion correction, the maximum error between the range fining at 65.165 m and the measuring value of interferometer is less than 500 μm. The standard error is 246 μm and the frequency spectrum resolution reaches 123 μm, quite close to the theoretical resolution. The research in this paper provides feasible reference for the large-length range finding of FMCW.