The invention discloses a biomass fertilizer retaining agent and a preparation method thereof and belongs to the technical field of fertilizer processing. Graphene sol, kieselguhr and porous starch are used as main materials, and graphene in the graphene sol is of a regular two-dimensional structure, has a large specific surface area and can be combined with a kieselguhr porous substance, so thaton one hand, with the combination of the two-dimensional structure of graphene and the three-dimensional structure of porous kieselguhr, the prepared fertilizer retaining agent is of a multi-dimensional spatial structure; meanwhile, as both the graphene and the kieselguhr are nontoxic environmental-friendly substances, and surfaces of prepared granules are coated by a membrane formed by the porousstarch, the solidness of the granules can be improved; meanwhile, with the secondary combination of the three-dimensional structure of the porous starch and the multi-dimensional structure of the granules, fertilizers can be adsorbed by the prepared fertilizer retaining agent through the three-dimensional structure, and soil moisture can also be absorbed, so that the fertilizer retention propertyof a biomass fertilizer can be improved, and testing data shows that the retention effect on total nitrogen in soil can be improved by 184% to the maximum.