Abstract: In order to continuously forecast the carbon content in a RH molten bath and control the carbon content at the RH terminal point,the off-gas flow rate was analyzed and the content of CO + CO₂ in the off-gas was monitored continuously. Based on carbon balance calculations,a decarburization mathematical model was established for the 250 t RH in a steel plant. Calculation results from the model showed that the precision of carbon content at the end point of RH treatment was ±5×10^-6 for ultra-low-carbon steel (wC ≤ 30×10^-6). When the content of CO + CO₂ in the off gas was lower than 5% in the late period of RH decarburization,the decarburization rate was lower than 10^-6 min^-1,so decarburization could be determined to finish. In combination with site technological conditions,the effects of the pressure drop platform and oxygen blowing operation on the RH decarburization rate were studied.