It is commonly known that the mechanism of fire retardancy of boric acid is by forming a coating or protective layer on wood surface at high temperature. However, this mechanism could not explain the phenomena in our previous work, in which boric acid influenced the pyrolysis of wood at much lower temperature. To understand better the fire retarding mechanism of boric acid on wood, in this paper the effect of boric acid on the pyrolysis and fire retardancy of wood was studied by thermal analysis and cone calorimetry (CONE). Basswood (Tilia amurensis Rupr.) was treated with boric acid, guanylurea phosphate (GUP) and GUP boric acid respectively. The treated woods were analyzed by thermogravimetry (TG/DTG), differential thermal analysis (DTA) and by CONE. It was found in TG that the weight loss of boric acid treated basswood was about 3 times of GUP treated wood or untreated wood at 165℃, the temperature at which GUP is stable. The DTA curve showed that boric acid treated basswood has an exothermal peak at 420℃, indicating the exothermal polymerization reaction of charring. The CONE result showed that boric acid and GUP had considerable synergistic fire retardant effect on wood. It is concluded that boric acid has fire retardant effect on wood through mechanisms other than coating or protective layer, it has different effects in the different stages of the combustion of wood, it catalysts the dehydration and decomposition of wood and promotes the charring of wood. The fire retardant synergism of boric acid and GUP is due to their functioning by different mechanisms.