The flame of basic oxygen furnace is the most important evident in the steel making process. This studies dividing the flame’s spectrum into two parts: the background spectrum and the characteristic atomic emission spectrum. Compared the figure of the characteristic atomic emission spectrum measured by spectrometer with the Gaussian function, the conclusion shows that the background spectrum could compensate the loss of the light intensity which due to the stimulated absorption of characteristic atomic. Based on the FES ( flame emission spectrometer) and spectrum in the BOF’s flame, the studies deduce a new relationship between the intensity of characteristic atomic spectrum and the temperature of the flame. The results indicates that the temperature measured by FES is inosculated to the temperature obtained by converter sublance comparatively.