Tunable diode laser absorption spectroscopy (TDLAS) has been an effective technology to measure the velocity of the combustion flow in hypersonic engine. The measurement theory is based on the relative Doppler shifts of the absorption transition for the upstream and downstream beams in the flow. The resolution of the TDLAS system for velocity measurement, which is defined as the minimum velocity change value that the TDLAS system can identify, is analyzed in three aspects: the structure of the beam path, the absorption transition from the laser, and the quantization of the digital signal sampling. The measured accuracy of velocity is also investigated based on the study of the resolution. An experiment was executed on a scramjet engine with a TDLAS system to validate the resolution and the accuracy of the measured velocity.