B4C/Cu functionally gradient material (FGM) is a promising candidate for plasma facing material (PFM) in fusion device,because it can mitigate the thermal stress caused by the mismatch of the coefficient of thermal expansion (CTE).Thermal Plasma spraying technology is supposed to be suitable for the B4C/Cu FGM preparation.In this work,a RF driven Ar/H2 thermal plasma is used to prepare the B4C/Cu FGM.In view that the characteristics of plasma are important for the preparation process,spectroscopic technology is used to diagnose the Ar/H2 plasma characteristics.The gas temperature is determined by comparing the simulated and experimental hydroxyl (OH) spectra.