Abstract: High-enthalpy arc heaters play an important role in the development of thermal protection materials and heat shield structures for entry vehicles because they are capable of producing longtime and representative flow environments. Owing to the large heat flux loading on the electrode, the erosion of the electrode is inevitable. Generally, high pressure water is used for cooling of the electrode. The arc heater may suffer serious damage caused by electrode leak, especially for hundreds or even thousands of seconds aerodynamic heating tests. Therefore, it is necessary to develop fast response diagnostic technique to monitor the operating status of the facility and determine the initial time of water leakage to avoid costly arc-heater failure. Because of the extreme conditions inside the arc-heater section, options for measurements of the test gases are limited, and optical spectroscopy-based measurements present a diagnostic opportunity. Optical Emission Spectroscopy (OES) is widely used for measuring gas parameters of high-temperature flow field because it is non-intrusive, high sensitive, and just constituted of simple instruments. In our studies, the 656.28 nm emission spectral line of the atomic hydrogen and the 777.19 nm emission spectral line of the atomic oxygen are utilized for routinely in situ monitoring the operating status and determining the initial time of water leakage at a high-enthalpy arc heater. According to the intensity ratio of the two emission spectral lines, the mass fluxes of the water leakage can be derived, which are 1.85~0.94g/s and 2.12~0.98g/s, corresponding to equilibrium temperatures equal to 6000~8000K and 5500~7500K under two different test conditions, respectively. The current test results of this study illustrate the feasibility and potential of the OES technology in high-enthalpy arc heater safety diagnosis, especially on the water leakage diagnosis.