The microcellular foaming of two kinds of polysulfones, polyphenylsulfone (PPSU) and polysulfone (PSU), were investigated in supercritical CO2. The expansion ratio, which ranges from 1.10-2.45 for PPSU and 1.10-3.72 for PSU obtained under different conditions, is not sufficient due to the limited solubility of CO2. To improve the porosity of foams, the effect of co-blowing agents on foaming was studied by molecular modeling and foaming experiments. Ethanol has a more favorable interaction with CO2, followed by water, acetone and ethyl acetate. Additionally, ethanol significantly increases the interaction of CO2 with polymer chains The expansion ratio of foams has an evident increase, and the foaming temperature window expands upon the addition of co-blowing agents. Consistently, the addition of ethanol has the highest expansion ratio, 5.02 for PPSU and 6.54 for PSU. Meanwhile, it broadens the foaming temperature dramatically, decreasing the value by 50 degrees C for PPSU and 70 degrees C for PSU.