This paper was aimed to evaluate process of APP/SiO2, which used Nano-crystalline cellulose (NCC) modified ammonium polyphosphate (APP) as anionic polyelectrolyte (a-APP), and cationic polyethyleneimine (PEI) modified Nano–SiO2 as cationic polyelectrolyte (c-SiO2). The flame retardant system was built due to the reaction of a-APP and c-SiO2. Polyelectrolyte composite of a-APP/c-SiO2 were then assembled on the surface of wood powder and HDPE composites. The effect of polyelectrolytes on wood-plastic composites (WPC) were investigated and the results showed that the flame-retardant property of WPC treated by polyelectrolyte was the best. The average heat release rate was 152.8kW/m2, the peak heat release rate was 352.2kW/m2, the total heat release was 108.5kW/m2, the limit oxygen index reached the maximum was 27.5%, compared with the WPC treated by APP, the elongation at break increased by 60.4%. After anionic and cationic polyelectrolyte treatment, making anionized a-APP and cationized c-SiO2 due to the charge interaction, in the WPC combustion process to form a dense, uniform WPC carbon layer, thereby reducing the heat transfer to the material inside, and increasing the flame retardancy of WPC composites
