A model of the oxygen content in the bell furnace is obtained by taking the furnace leakage flow into consideration based on the method of mechanism modelling. On the basis of mass conservation law, the relationship between the oxygen content in the bell furnace and the pipeline flow are presented while the leakage of the bell furnace and its effect on oxygen content is deduced according to Bernoulli's equation. Secondly, the oxygen content process model of the bell furnace is appropriately simplified to obtain the system transfer function, and the effect of the time delay is considered to modify the transfer function of the process model. The experiment is then applied to a real bell furnace, the step response and a typical craft response are measured and compared with the simulation. In the typical craft response experiment, the average error between the simulation and the experiment is 2.1819 mV measured in voltage when the oxygen content is above 0.01 %. The results indicate that the process transfer function based on first principle modeling is significant when the oxygen content is above 0.01 %, and can be further applied in designing oxygen control method both in close loop and open-loop control system.