An appropriate sidestream is the key to improve the yields and purities of products in the cryogenic air separation process. The sidestream of a full tower internal thermally coupled air separation column is characterized. Sensitivity analysis indicates that heat duty distribution, purities, and yields of the products are strongly influenced by the withdrawn position and the flow rate of the sidestream. Optimization models are proposed by adopting the minimum flow rate of the low-purity liquid nitrogen product and with maximum flow rates of the oxygen product or nitrogen product as optimization target. The optimization research allows selecting the optimum operating conditions with different production requirements.