Abstract: In this paper, we are using hyperbranched polyethyleneimine (PEI) modified ammonium polyphosphate (APP) as flame retardant (PEI/APP) and phenol formaldehyde resin (PF) as an adhesive to prepare oriented strand boards. Using DSC to explore the influence of PEI/APP on the curing process of phenol-formaldehyde resin, PEI/APP was able to form intermolecular hydrogen and imine bonds (C=N) with PF to promote the curing process of the phenol-formaldehyde resin, so that the peak temperature of curing was lowered to 71.6℃, and the end of curing temperature was lowered to 74.1℃. Using mechanical property analyzer, digital image correlation and X-ray computed tomography to understand the mechanical properties of oriented strand boards (OSB) and its enhancement mechanism. Compared to the oriented strand board with 9% APP addition, the modulus of elasticity of OSB with 9% PEI/APP addition increased by 21.1% and 20.7% in the major (L) and minor (H) directions, respectively; the major and minor modulus of rupture was increased by 10.8% and 19.6%, the internal bond strength was increased by 6.9%, and the thickness swelling of the absorbed water was reduced by 37.9%. DIC shows that when OSB is loaded in bending, strain accumulation mainly occur in the top and bottom layers and then propagate along or across the central layer. Structural changes in the top and bottom layers of major specimens was either strand delamination or snap off, while only snap off is found in minor specimens. X-CT test results show that PEI/APP and PF resins are widely spread in the surface layer and core layer of OSB. Therefore, the hyperbranching modification improves the interfacial bonding between APP and the large wood shavings and PF resins, which results in making strain transfer along the center layer more pronounced. The center layer absorbs the energy of the load, which im-proves the bending strength of the OSB. The flame retardancy of OSB was tested using LOI and cone calorimetry. Its limiting oxygen index increased by 27.9% and its peak heat release rate decreased by 14.2%. Hyperbranched modified APP could induce OSB dehydration charring to produce a dense and continuous expanded char layer, and at the same time produce NH₃ to realize synergistic flame retardation in both gas and condensed phases. Flame-retardant and high-strength oriented strand board has a very wide application prospect in the field of green building, furniture and transportation applications.