The invention relates to a preparation method for a bamboo fiber sandwich composite plate, and belongs to the field of plate preparation. The method is characterized by comprising the following steps:phenol, water, paraformaldehyde and a sodium hydroxide solution are added into a flask, the pH value is adjusted, and a reaction product is cooled and discharged to prepare expandable phenolic resin;an original bamboo fiber material modified by a silane coupling agent is added, then a surfactant, a foaming agent, and a curing agent are added in sequence, mixing is performed uniformly, a resin mixture is poured into a mold, and foaming curing is performed; and artificial roll coating sizing is performed: the upper and lower surfaces are separately provided with a layer of thin bamboo plate, after coating is performed, closing is kept for 10-15min, cold pressing is performed at a normal temperature, and therefore the composite plate is prepared. According to the invention, through the improvement of the preparation process, the prepared composite plate has enhanced compressive strength, and improved longitudinal bending strength and transverse bending strength, and the preparation process is simple for operation, and suitable for industrialized promotion and application.