This paper investigates the characteristics of the cryogenic rectification column as part of an ASU in IGCC plant. The mathematic model of the low pressure column and the condenser-reboiler is developed. Then, the static characteristics, such as the concentrations of nitrogen, oxygen and argon, the temperature and pressure in column are discussed. Starting from the steady-state, dynamic simulations are carried out as follows: the oxygen-rich liquid mass flow, the liquid nitrogen mass flow, the air from expander mass flow and the product oxygen mass flow are changed, and the dynamic characteristics of the system are presented.