Aiming at the accurate extraction of Doppler frequency of laser Dopple r length measuring instrument,an adaptive sampling frequency Doppler signal processing algorithm based FPGA is pr oposed.The adaptive selection of sampling frequency is realized by tracking Doppler frequency,t he accuracy of low speed measurement is improved and the velocity range of the system is expanded.The FF T-FS spectrum thinning algorithm and the ratio correction algorithm are combined to effectively reduce the measurement error and further improve the accuracy of the Doppler frequency extraction.Finally,the feasibili ty of the algorithm is verified by a variety of length measurement experiments,a nd the relative error of the measurement results is less than 0.1%.