Abstract: Gas temperature and species number density are the key parameters to quantitatively assess the arc-heated wind tunnel operation and flow quality. Conventional techniques meet great challenge in high enthalpy flow diagnostics for arc-heated facilities under prolonged operation at high temperatures. Based on the local thermodynamic equilibrium plasma assumption, this paper presents in-situ diagnostics for the dissociated air (＞5000K) in the arc heater by using laser absorption spectroscopy of atomic oxygen at the wavelength of 777.19nm. The gas temperature and the number density of atomic oxygen are measured under two operation conditions of H₀=15.8MJ/kg and 17.4MJ/kg, respectively. The average temperatures are 5843K and 6047K, corresponding to 5950K and 6335K from charts for high temperature equilibrium flow properties of air. The number density of atomic oxygen is within (1.1~1.2)×10¹⁸cm^-3 and is in consistency with the calculation via NASA-CEA program, while the number density of atomic oxygen (⁵S₂⁰) is within (1.0~1.5)×10¹⁰cm^-3. This work demonstrates that the laser absorption spectroscopy is applicable for high enthalpy flow diagnostics in the arc-heated wind tunnel as a new technique.