The Mars exploration mission is important to the national deep space exploration project. The successful landing of “Tianwen” on Mars in 2021 is a big step forward in Chinas space exploration and research on Mars. The Martian atmosphere is mainly dominated by carbon dioxide, and the surface pressure is much lower than the Earths. Due to the differences in atmospheric composition and entry orbit, the thermal environment of Mars entry vehicles is very different from that of Earth space vehicles. The entry process involves a non-air high-speed flow, leading to serious aerothermal heating problems. The arc-heated wind tunnels are used to simulate the aerothermal heating environment for Mars entry, which is a key step in validating the thermal protection system design for Mars exploration. The -high-temperature flow simulated by the arc-heated wind tunnel includes a dominated CO2 dissociation reaction, and dissociation species such as CO interact with the thermal materials of the Mars entry vehicles, which would cause a catalytic effect and significantly affect the aerothermal environment. In this paper, the mid-infrared quantum cascade laser absorption spectroscopy is carried out for in-situ and quantitative measurements of the flow characteristics of ground-based simulation for Mars entry in the arc-heated facility. The in-situ diagnosis in the arc-heated facility utilizes the spectral line of CO with a central wavelength of 2 212. 625 cm-1 (ν″=0, R(19)) near 4. 5 μm, to achieve a high signal-to-noise ratio measurement of the ground-based flow field. A single-line direct absorption spectroscopy obtains the static temperature and CO mole fraction of free stream in the arc-heated wind tunnel. Under a typical condition for Mar entry, the static temperature and CO mole fraction of freestream remain stable throughout the operation, showing good stability in the arc-heated flow field. Six repeated experiments show that the static temperature and CO mole fraction are in the range of （1 757±69） K and （0. 189±0. 027） respectively, with fluctuation of less than or equal to 3. 9% in flow temperature and fluctuation of less than or equal to 14. 3% in CO concentration, demonstrating great repeatability for the ground-based simulated flow field. The developed mid-infrared laser absorption spectroscopy diagnostic technology in the arc-heated wind tunnel can provide refined measurement capabilities for research of the aerothermal environment and ablation-catalytic properties of the thermal protection materials of the Mars entry vehicles.