Rigid polyurethane (PU) microcellular foams with high expansion ratio and uniform cell morphology can be prepared directly using supercritical carbon dioxide (CO2) as blowing agent. The in-situ viscosity measurements during PU polymerization in high-pressure CO2 were adopted to investigate the melt strength change as well as the PU foaming windows. It was found that a certain viscosity window existed for available foaming, which changed little with the foaming temperature or CO2 pressure, but the required reaction time to reach this viscosity foaming windows changed a lot because the existence of CO2 significantly delayed the viscosity rising. Furthermore, the viscosity foaming windows varied with different PU formula systems, e.g., about (300-400) Pa.s for C-1/PAPI-135C system which could be foamed with cell size smaller than 10 mu m, and about (245-400) Pa.s for R090/PM200 system which was foamed with bigger cell size of (20-40) mu m.