In order to meet the needs of length on-line measurement in the process of cable production, a cable length on-line meter calculating system based on laser Doppler velocimetry was designed. The basic principle of laser Doppler velocity measurement was described, a compact double-beam double-scattering optical path was designed. A filter band adaptive selection algorithm based on finite impulse response (FIR) filter was proposed, which effectively filtered out the base signal and partial noise, reduced the influence of Doppler signal noise and improved the signal-to-noise ratio. The use of energy barycenter correction technology for spectral correction reduced the measurement error of the system. The experimental device was set up, and the contrast experiment was carried out. The experiment results show that the system could measure the length in real time and accurately, and the relative error is less than 0.1%.