In the Fourier transform spectrometer (FTS), obtaining the optical path difference of each sampling points accurately is the key of data processing. The velocity fluctuation of moving mirror will cause equal-time sampling error, and traditional equal optical path difference sampling method can not be used for visible wavelengths. The influence of speed fluctuation is analyzed and a method to calculate the optical path difference of each sampling points based on fitting algorithm is developed. The method is verified by processing the interference signal of visible light with large velocity fluctuation. The experimental results verify its correctness and effectiveness. This method can also be used in any kind of Fourier transform spectrometer.