Abstract: Simulation of the aerothermal environment for Mars entry by the arc-heated plasma wind tunnel is a key to develop the thermal protection system (TPS), since the thermal environment for Mars entry is very different from that of earth reentry. In this study, in-situ quantitative measurements of the radiation characteristics of CO₂ plasma and the flow parameters of the simulating freestream for mars entry were carried out by using emission spectroscopy and mid-infrared quantum cascade laser absorption spectroscopy. The electron temperature of the CO₂ plasma in the arc heater and the static temperature and CO concentration in the freestream were obtained. The emission spectroscopy and laser absorption spectroscopy measurements show that the arc heater has good stability for long-term operation and a capability for repeatable simulation of the flow parameters. The developed spectral diagnostic technology in this study provides an effective measurement method for studying the flow characteristics of the Mars entry aerothermal environment.