The thermal stabilities of bio-based polyurethane (PU) foams made from liquefaction bio-polyols with and without solid residue were analyzed by thermogravimetric analysis (TGA) and Fourier transform infrared spectrometry (FTIR). Yaupon holly (Ilex vomitoria) was subjected to microwave liquefaction at different reaction temperatures to characterize the variations of bio-polyol and solid residue with temperature. The results indicated that the solid residue decreased when the temperature increased from 120 °C to 160 °C, while it increased slightly when further increasing temperature to 200 °C. The hydroxyl number decreased with increased reaction temperature. The TGA of PU foams demonstrated that the use of liquefaction bio-polyol with and without solid residue to produce PU foam increased the thermal stability of biofoams as compared with petro-based foam. Moreover, the presence of solid residue in bio-polyol enhanced the thermal stability of biofoams. The FTIR analysis of PU foams suggested that the solid residue had a negative effect on the formation of urethane bonds.