Abstract: The flow surrounding a hypersonic vehicle downstream of a strong shock exhibits pronounced high-temperature thermochemical nonequilibrium because of intense aerodynamic heating. Its evolution is governed by the competition and coupling among multiple microscopic energy-transfer and chemical-reaction processes, highlighting the urgent need for additional experimental data to validate and refine existing models. A spectroscopy measurement platform was built for cylindrical flow based on Ф800 high-temperature low-density shock tube. Molecular and atomic emission spectra were measured under a post-shock equilibrium air inflow condition corresponding to an incident shock velocity of approximately 6 km/s, providing the spatial distributions of characteristic temperatures, species number densities, and atomic excited-state populations within the shock layer. Based on the conventional two-temperature model, a three-temperature thermochemical nonequilibrium framework was developed by separating vibrational energy from electronic-state and electron translational energy, and was employed to evaluate the nonequilibrium characteristics along the stagnation streamline. The results show that, compared with the two-temperature model, the three-temperature model predicts a longer thermochemical relaxation distance and slower excitation processes of vibrational and electronic energy modes. The experimentally inferred electron temperature agrees better with the three-temperature model, indicating that an independent description of the electronic energy mode is important for improving the prediction of shock-layer nonequilibrium processes under the present conditions. However, the measured vibrational temperature of the N₂⁺(B) state remains lower than the electron temperature throughout the shock layer, suggesting that current multi-temperature models may overestimate the energy transfer rate from translational-rotational modes to vibrational modes. Furthermore, the analysis for spectra of N/O atomic multiplets reveals that the populations of excited states in the core region of the shock layer generally follow Boltzmann distributions characterized by the electron temperature predicted by the three-temperature model. In contrast, near the shock front and in the near-wall region, the upper excited-state populations exhibit premature establishment and delayed decay, respectively, indicating significant electronic-state nonequilibrium and non-Boltzmann population distributions in both the strongly compressed excitation region and the cooling recombination region.