Mold flux is an essential component for quality casting of steel as it controls heat transfer, lubricates the mold, and insulates the steel both thermally and chemically. However, because mold flux and steel interact during long sequences, the mold flux composition must be closely monitored to prevent undesirable defects. This paper proposes a high- temperature fiber-optic sensor to detect the Raman spectra of mold flux at 1400°C in real time that is suitable for online analysis. The experimental results demonstrate the viability of employing Raman spectroscopy to monitor the chemical composition of a mold flux under high-temperature conditions during continuous casting.