The end-point control of basic oxygen gurnace (BOF) consists of temperature control and element control. The flame of converter can reflect the decarburization rate and operation parameters of converter. The atomic emission spectra of alkali metals such as kalium and sodium exist in the boiler flames, the temperature of flame can be calculated with the spectral line inversion technique. Two-color method, three-color method and two-line method are used to calculate the flame temperature. The characteristic lines are extracted from the fitting baseline and the wavelet ridge line fitting baseline. Characteristic spectral lines are fitted with Gauss function and Lorenz function. The results show that the radiation thermometry is sensitive to the spectral lines, the reasonable band is necessary to measure the flame temperature effectively and accurately. Two-line method is limited by mathematical model of characteristic spectral line, transition probability of spectral line, degenerate degree of energy level, optical thickness of flame. High resolution spectrometer is necessary to measure the converter flame temperature.