Flame of basic oxygen furnace is the most important evident in the steel making process. The flame spectrum is divided into the background spectrum and the characteristic atomic emission spectrum. Compared with the figure of the characteristic atomic emission spectrum measured by spectrometer with the Gaussian function, the conclusion shows that the background spectrum can compensate the loss of the light intensity due to the stimulated absorption of characteristic atomic. Based on the flame emission spectrometer (FES) and spectrum in the basic oxygen furnace (BOF′s) flame, a new relationship between the intensity of characteristic atomic spectrum and the temperature of the flame is deduced. The results indicate that the temperature measured by FES is inosculated to the temperature obtained by converter sub-lance comparatively.