The flame retardancy of poplar plywood was studied through the indexes of heat release rate, total heat release, effective combustion heat, CO formation rate, CO2 formation rate, and total smoke release. The results showed that the heat release rate, total heat release, effective combustion heat, CO2 formation rate, and total smoke release of poplar plywood reduces and the CO formation rate increases with nano-zirconia and magnesium hydrogen phosphate respectively, but the flame retardant effect can' t achieve the goal; the composite flame retardant of nano-zirconia and magnesium hydrogen phosphate have synergistic effect on flame retardancy of poplar plywood in the process of combustion; the poplar plywood stops burning after burning for 190 s; the retardancy effect of composite flame retardant is better than that of single flame retardant.