Non-contact on-line measurement of gas velocity is of great significance in the experimental study of high-speed flow field such as engines.Based on the Doppler frequency shift effect of absorption line in gas absorption spectrum technology,a high-speed airflow velocity measurement system is designed and constructed,and the speed of airflow accelerated by Laval nozzle is measured online in real time.In the experiment,the characteristic absorption line of H2O near 1392 nm is selected,and data acquisition and processing are conducted based on LabVIEW to obtain real-time velocity.On this basis,the speed measurement limit of the system is analyzed,which lays a foundation for further optimization of the follow-up system and development of the overall device.