Abstract: A consolidated mathematical model and its computing method for a universal modeling column were proposed with Harmens state equation and the gas-liquid equilibrium theory of O₂-Ar-N₂ mixture. Based on them, a calculating model of an air separation unit (ASU) system was developed and the rectifying process was analyzed. The result of an application to No.5 ASU in BaoSteel made in APCI shows that the model and the computing method can accurately forecast the variation of operating parameters of ASU in the rectifying process.