The work aims to study the effect of silane coupling agent KH550 content on the properties of wood flour/P34HB composite packaging materials, increase the bonding strength of wood flour modified by KH550 with poly (3-hydroxybutyrate-4-hydroxybutyrate) (P34HB), and improve the mechanical properties and interfacial compatibility of the composites. Modified wood flour/P34HB composites were prepared with KH550 as modifier and wood flour and P34HB as raw materials by blending hot pressing process. The effects of different mass fractions of KH550 on interfacial compatibility, mechanical properties and thermal properties of composites were studied by morphology observation, Fourier transform infrared spectroscopy (FTIR), thermal analysis (TGA) and mechanical properties analysis. After the addition of KH550, the interfacial compatibility of the composites was improved. FTIR showed that, KH550 had been grafted into wood flour; the proper amount of KH550 improved the thermal stability of the composite; the storage modulus of the composite increased; and the mechanical properties of the composite were also improved. In addition, the best addition amount of KH550 was obtained, and the mass fraction was 0.5%. KH550 not only improves the compatibility of wood flour with P34HB, but also enhances the mechanical and thermal properties of composite packaging materials.