Simultaneous online measurement of gas concentration and velocity can be realized by tunable diode laser absorption spectroscopy (TDLAS) technique and optical signal cross correlation method. The fundamental and relative factors of gas concentration and velocity measurement are described in the present paper. The spectral lines of NHa used for gas sensing at communication band in near infrared range were selected and analyzed by the calculation based on the HITRAN database. In the verification experiment, NH3 and N2 were mixed by two mass flow meters and sent to flow through the quartz tube 0. 016 m in inner diameter and 1 m in length at normal temperature and pressure. The spectral line located at 6 548.7 cm^-1 was scanned at high frequency by the diode laser of 15 MHz linewidth and 1 cm^-1 tunable range with no mode hoppings. The instantaneous NH3 absorbance was obtained using direct absorption method and the gas concentration was calculated. At the same time, the non-intru sive optical absorption signal cross-correlation method was utilized to obtain two concentration signals from two adjacent detectors mounted along the gas tube. The corresponding transit time of gas passing through the detectors was calculated by crosscorrelation algorithm, and the average gas velocity was inferred according to the distance between the two detectors and the transit time. The relative errors were less than 7% for the gas concentration measurement, and less than 10% for the gas velocity measurement. Experimental results were proved to be of high precision and good repeatability in the lab. The feature of fast response and capacity immune to the in-silu disturbance would lead to a potential in industry application for the real time measurement and control of gas pollutant emission in the future.