The measurement error caused by steel slag, molten steel surface mirror effect can be avoided by molten steel temperature detection on the basis of infrared thermal imaging and colorimetric thermometry. The temperature of molten steel can be obtained by calculating the forward radiation probability of fluctuation, infrared image gray level distribution and so on, and confirming the optimal sampling area. Giving the temperature change curves of molten steel and improving data accuracy and continuity, are conducive to the realization of precision steel making.