In the preparation of wood-plastic composites (WPCs) with flame retardant treatment, the addition of insoluble ammonium polyphosphate (APP) can increase the physical and mechanical properties of WPC. To analyze the effect of APP on WPC during the hot pressing process, we calculated the apparent activation energy of wood fibers at temperatures ranging from 170℃ to 190℃ using a Coats-Redfern method. We analyzed the changes of functional groups in treated and untreated fibers after hot pressing by Fourier transform infrared spectroscopy (FT-IR). Finally, wood fiberboard and wood-plastic plywood were prepared to verify our conclusions. Results showed that: 1) the apparent activation energy of flame retardant fibers was lower than that of blank fibers; 2) the amounts of carbonyl and methyl groups and the number of ether linkages in treated fibers changed; 3) flame-retardant wood fiberboard had higher bending strength than wood-plastic plywood, but the plywood had a higher dry strength. From these results we can conclude that the addition of APP increases surface activity of fibers and interface compatibility between wood and plastic under hot processes. In general, WPCs with the addition of APP have improved physical and mechanical properties.