Abstract: In order to continuous on-line measure the temperature of liquid steel in RH vacuum tank, timely and accurately calculate the temperature drop rate at all stages of refining process, instructing the temperature adjustment operation，the temperature measuring device was designed and developed based on infrared thermal image and colorimetric temperature measurement technology. The industrial test of the device was carried on 120 t RH refining furnace and 298 heats’ data was obtained. The temperature value corresponding each stages of steel fluid circulation begin, end of decarburization, end of aluminum adding, end of powder injection and endpoint obtained by the device was contrasted with the manual measurement results measured by means of thermocouples. The comparison results show that the temperature difference between this device and manual measurement varies in the range of －5.0 ℃ to 6.3 ℃. Among them, the proportion of the difference fluctuation in range of ±3 ℃ is 92.1 %,within ± 5 ℃ up to 98.7 %. At present，the technology has been successfully online applied in 120 t RH refining furnace.

Key words: molten steel temperature, RH refining, infrared colorimetric temperature measurement, continuous temperature measuring