The high quality single-phase solution of CO 2 / Polystyrene was achieved, by analyzing the influential factors for polymer microcellular foaming extrusion. The curve of pressure distribution along the barrel was determined. The axial position of gas-injecting port on the barrel was chosen form the results of stable foaming, and the number of gas-injecting ports in the circumference of the barrel was determined from the CO 2 solubility in polymer. The effect of the screw rotation speed on CO 2 solubility was studied, and the effects of pressure difference between the gas and the polymer melt on gas-injecting process and on the foaming stability were investigated. The influence of the gas temperature before injection on the single-phase of CO 2 /Polystyrene solution also was studied.