Environmentally friendly materials with high biomass content have always attracted attention. In this work, wood flour (WF), a renewable and environmentally friendly substance was modified by castor oil-based polyurethane prepolymer (COPU) and urea-formaldehyde resin, respectively, to prepare thermoset composites with high biomass content and excellent interface compatibility. A special intensive mixer designed and produced by our laboratory was used for the first time to mix the biobased composites with high viscosity. The WF biomass content of the composites was as high as 70 wt% and the composites were prepared by melt blending and hot pressing. In effect, 70 wt% of WF biomass and 17.5 wt% of castor oil biomass were used to prepare wood-plant oil composite with almost 87.5 wt% of biomass. Through infrared spectroscopy (IR), emission scanning electron microscopy (SEM), and thermodynamic characterization, it was found that compared to composite materials which were prepared using WF and polypropylene (PP) or deactivated COPU, the interface compatibility and thermal stability of the composite materials obtained by WF and COPU or UF were significantly improved. The mechanical properties of the obtained composite material were attractive, and the bending strength and bending modulus of thermoset composites prepared using WF and COPU were as high as 69.2 MPa and 3765.8 MPa, respectively. The comprehensive results revealed that the WF-based thermosetting composite material with high biomass content as well as excellent performance and its application in composite panels are worth looking forward to.