Based on the laser Doppler effect, real-time and on-line measurements of velocity in high-speed flows can be realized using tunable diode laser absorption spectra (TDLAS). The fundamental theory and the method to measure the velocity are presented. A dual-beam system used in the experiments is described. The water vapor absorption feature at 1398 nm is utilized to detect the flows generated within a wind tunnel. The DFB laser is quickly tuned at a rate of 10 kHz to obtain the spectra of the transient flows by using scanned-wavelength direct absorption spectroscopy. The ways to calibrate the frequency and calculate the Doppler shift (DS) are also introduced. The measured velocity is compared with the calculated velocity, and they are in good agreement with each other, which proves that the way combining DS with TDLAS to measure the velocity in high-speed flows is reasonable. The system capability and possible factors to induce measurement deviations are analyzed to improve the system performance in the future.