Typically, pure lignin is considered to be limited in flame retardancy and phosphorylation of lignin or combination with other effective flame retardants is required to help polyurethane achieve flame retardancy. Three types of lignin, including sodium lignin sulfonate (LS), alkaline lignin (AL) and enzymatic hydrolysis lignin (EHL), were used exclusively to significantly improve the flame retardancy and thermal stability of PU foams. The PU foam coated with AL or LS was able to show significantly improved flame retardancy due to the increasing density and swelling of coal, radical scavengers and inert gas dilution to protect the PU foam from fire and heat, and the limited oxygen index could reach up to 28.4. The thermal stability of the coated PU foam was significantly improved in that 44 percent by weight remained at 900 ° C., compared with almost 0 percent by weight that remained with the control PU foam. The flame protection mechanism of lignin / PU foam was attributed to the fact that lignin could change the thermal degradation and the burning behavior of PU foams in the condensation phase (carbonization), gas phase (inert gas dilution), radical quenching (phenolic groups) and SN synergism. Our research has proven that lignin alone can help make PU foam completely flame retardant without phosphorylated modification or combination with other flame retardants.