Abstract: The flammability and pyrolysis characteristics of ammonium borate trihydrate and diammonium hydrogen phosphate in poplar veneer were analyzed by using of the limiting oxygen index, horizontal and vertical burning tests, thermos-gravimetric analysis (TGA) and scanning electron microscope (SEM). The results showed that the flame retardancy of diammonium hydrogen phosphate was better than ammonium borate trihydrate. The obvious synergistic effects between diammonium hydrogen phosphate and ammonium borate trihydrate were found, the oxygen index and the remaining amount of carbon residue could approach to 61% and 53.5%, respectively, using 20% concentration treatment, it also exhibits superior flame retardancy at horizontal and vertical burning tests. Moreover, the onset decomposition temperature, peak decomposition temperature and activation energy of each stage were reduced, with the addition of the composite flame retardant, the SEM images indicating protective chars formed from the treated wood, terminating the transmission of heat and oxygen.