The continuous production of polystyrene microcellular foams with supercritical CO2 was achieved on a two-stage single-screw extruder. Simulations related to the foaming process were accomplished by modeling the phase equilibria with the Sanchez-Lacombe equation of state and combining the equations of motion, the energy balance, and the Carreau viscosity model to characterize the flow field and pressure distribution in the die. The position of nucleation in the die was determined from the simulation results via a computational fluid dynamics code (FLUENT). Experimental parameters were selected according to the T-g and phase equilibria. The effects of CO2 concentration and die pressure are explored. Below the solubility limit, higher CO2 concentrations lead to smaller cell size and greater cell density. With an increase of die pressure, the cell size decreases and the cell density increases.