As a high-precision measurement tool, femtosecond laser is superior to traditional laser technology and has been widely used in industrial production, aerospace, scientific research and other fields. The method of frequency sweeping and optical sampling has greatly improved mechanical vibration and slow scanning speed, which has great significance for improving the absolute ranging performance of femtosecond laser. A large-scale distance measurement method using femtosecond laser is proposed based on the principle of frequency sweeping and optical sampling, and research is performed on the measurement principle, interference spectrums and demodulation algorithm of this technology. Firstly, according to the generation principle of mode-locked femtosecond laser and the piezoelectric effect of piezoelectric ceramics, the method of continuous scanning repetition frequency of femtosecond laser was introduced. On this basis, the traditional optical sampling principle was used to explain the range measurement principle of optical sampling method, and the influence of the fiber delay line length on the scanning range was deduced and discussed. Then, a distance measurement system using femtosecond laser was set up, long distance measurement experiments were carried out on a linear guide, and the He-Ne reference light path based on Michelson interference principle was designed. The group refractive index of air was modified according to the experimental environment, the influence of measured distance on the spectrum of interference fringes was analyzed, and the scanning ranges at different target positions were measured. In the range of 504 m, the scanning range increased from 056 to 112 mm, and it fully proved the importance of fiber delay line for improving large-scale ranging capabilities. Periodic scanning frequency could generate cross-correlation fringes, the real time frequency variations and sampling multiplication factors were calculated by performing Hilbert transform processing on the interference spectrums, and the distance information was obtained. In addition, in order to reduce the time delay error of the system and improve the accuracy of the measurement, the difference principle was used to improve the algorithm. The experimental results show that, by contrast, the improved algorithm based on distance difference is used to process dataand the performance results are better. After the algorithm is improved, the measurement accuracy of the system is improved from 11 to 4 μm in the range of 50 m, and the relative accuracy is improved from 22×10-9 to 8×10-8, which indicates that the accuracy is significantly improved. The repeatability measurement data is analyzed and compared with the results of incremental laser interferometers, the standard deviation of measurement error is improved from 10 to 2 μm, and the maximum relative stability is improved from 2×10-9 to 4×10-8, which indicates that the stability is significantly improved. Therefore, the method has excellent capability of large-scale distance measurement, the potential of achieving high-accuracy, large-scale and fast absolute range measurement simultaneously, and great prospects in the field of precision spectrum measurement in the future.