Several polyurethane flame-retardant coatings,blended with different foaming agents,were applied to Pinus massoniana plywood.To evaluate the effect of the different foaming agents and the addition of nano-SiO2,the flame retarding properties of the treated plywood were tested with respect to an oxygen index value and heat release rate,respectively,by HC-2 oxygen index equipment and an HRR3 heat release rate system.The results showed that:1) the most effective foaming agent was the compound of carbamide and dicyandiamide（blended as the most effective ratio at 1∶3）,and 2) When the addition of nano-SiO2 was increased,the oxygen index of plywood increased while the highest heat release and the total heat release amount in 2 minutes decreased..However,all indexes changed little when the addition of nano-SiO2 exceeded 3%,except a dramatic increase in the cost.