Abstract: Ethylene-ethane rectifier is a typical complicated close-boiling fractionation system，which includes product side-draw，several middle reboilers and strict product requirement. As a typical multivariable system，close-boiling fractionation system is very complicated，and these variables are seriously connected each other. Dynamic model establishment and system simulation will help to coordinate the relations between variables and to implement advanced control plan. However，the complication of actual system leads to many problems in the process of obtaining exact mechanic model and solving related mathematical equations. For example，the accuracy of enthalpy and equilibrium constant can greatly influence the dynamic simulation of close-boiling fraction. An ethylene distillation column was selected as a case to propose strategies dealing with these problems better. BWR state equations were used to calculate the enthalpy and equilibrium constant，the model accuracy was improved，and the enthalpy derivatives in the energy equation were reasonably simplified. Simulation results showed reasonable computational efficiency and accuracy. Such improved model for ethylene distillation column has a high precision，and can predict the dynamic behaviors better.