The technical scheme of the invention provides a preparation method of a bio-composite board. The method comprises the following steps: firstly, adding an adhesive into plant fibers, uniformly mixing,and drying the mixture to obtain adhesive fibers; softening veneers, soaking the veneers with an adhesive, and drying the veneers to obtain adhesive veneers; uniformly spreading the adhesive fibers on the adhesive veneer to form an adhesive fiber layer, and rolling up the material to obtain a biological composite coiled material; and finally, flattening the biological composite coiled material toobtain the product. The preparation method in the embodiment of the invention is simple in process; biomass resources are utilized, and the prepared bio-composite board is excellent in performance. The bending strength of the product reaches 35 MPa, and the impact strength reaches 9.82 kJ/m<2>.