The non- contact online temperature measurement of the flame at the converter mouth during the basic oxygen steelmaking process is investigated. Multi- spectral radiation thermometry based on the least squares method in which no emissivity model is needed is adopted for the flame temperature measurement. A flame radiation signal collection system at the converter mouth is constructed by taking the Ocean Optics miniature spectrometer USB4000 as the beam split element. The responsivity of the proposed system is obtained by adopting the radiation of the liquid steel in the converter whose temperature is close to that of the flame at the converter mouth for the system calibration. Temperature of the liquid steel during tapping is measured using spectral data of different wavelength bands with responsivity obtained by the improved calibration method and the halogen lamp method, respectively. The results show that the improved calibration method can achieve the best performance in the wavelength band of 600~740 nm. The online flame temperature measurement at the converter mouth is carried out with the proposed method, and the variation in the flame temperature during the steelmaking process is obtained. The results show that the proposed method can not only indicate the impact of the converter operation on the flame temperature at the converter mouth, but also offer a useful auxiliary way for the operators to control the temperature of the liquid steel in the converter.