Many parameters can be used to represent the flame resistivity of PURFs, and char-forming property in combustion process is one of the most significant one , which can be evaluated by the char residue at different temperatures. On one hand, higher char residue indicates that less PU macromolecule chains will be decomposed at this temperature. On the other hand, higher char residue also suggests that PU macromolecule chains are less likely to be oxidized at this temperature. In other words, the stabilization of PU at high temperature is normally favored. PURF with good char-forming property is advantageous as construction insulation material. First, char-forming effect will hamper the decomposition of PURF at high temperature, thus the heat release from PURF will be decreased. In addition, good char-forming property can lead to the formation of char layers with excellent thermal insulation, protecting the inner part from the fire. Furthermore, PURF after combustion and char-forming still has relatively complete cell structure with low thermal conductivity, which can prevent the collapse of building at high temperature. For these reasons, good char-forming property is indispensable for the safe application of PURF as construction material.