Based on the principle of distance measurement of laser frequency modulated continuous wave (FMCW), a distance measurement system of dual optical path FMCW is set up, which replaces the traditional linear frequency modulation with sinusoid frequency modulation so as to improve the sweep frequency. The principle of distance measurement of FMCW under sinusoid frequency modulation is also studied, the resampling formulas of sine sweep is derived, and the signal synthesis method to increase the sweep bandwidth and the quadrature modulation method to eliminate the stitching error are put forward. The experimental result shows that after the quadrature modulation method is used to splice the equal-light frequency resampling correction signal, the frequency resolution reaches to 127 μm, very close to the theoretical resolution. It is no more than 203 μm of the error and less than 194 μm of the standard deviation compared with the reference interferometer within the range between 3.3 and 4 meters.