Abstract: Arc-heated facilities play an important role in the development of thermal protection materials and heat shield structures for entry vehicles. Owing to the intensive heat fluxes generated by the arc root at the electrode surface, the erosion of the electrode is inevitable and there exists a risk of the cooling water leaking from the electrodes due to the accumulation erosion of the electrode material. It is well acknowledged that the arc heater can be severely damaged due to the late leak detection. Therefore, the development of fast response and accurately quantitative diagnostic techniques for leak detection of cooling water in an arc heater would be highly beneficial. Due to the harsh conditions inside the arc heater, especially for the low-enthalpy arc heater (total enthalpy range:2~12 MJ/kg), choices for the diagnostic methods are limited. Optical emission spectroscopy (OES) is a powerful tool to probe atoms and molecules which has been used in many areas such as chemical kinetics, non-equilibrium radiation, and so on. This paper presents an experimental investigation of the application of OES in the leak detection of cooling water for a low-enthalpy arc-heated wind tunnel (FD-04). According to the spectral characteristics of the high-temperature flow field in the FD-04 arc heater, the 777.19 nm emission spectral line of the atomic oxygen (O_777.19) is utilized for monitoring the operating status and determining the leakage of the cooling water. A relative intensity determination method for the O_777.19 emission is proposed and water leakage failures are successfully diagnosed under two test conditions at total enthalpy values of 11.6 and 9.8 MJ/kg. A solution is proposed that the diagnostic technology keeps high enough sensitivity for multiple total enthalpy conditions. This work demonstrates the potential of using the OES technique for leak detection of cooling-water in a low-enthalpy arc-heated facility.