The invention discloses biocharcoal-based slow release fertilizer. The biocharcoal-based slow release fertilizer is prepared from biocharcoal and chemical fertilizer with the mass ratio being 20:(5 to10). The biocharcoal-based slow release fertilizer disclosed by the invention has the beneficial effects that agricultural and forest wastes such as straws with wide distribution, large yield and lowcost are adopted as biomass, has rich pore channels, is easily decomposed, is not deteriorated after being absorbed, is wear-resistant, and also has excellent adsorbing and ion-exchange properties, the fertility, the structure and microorganisms and the like of soil can be effectively improved, and the soil quality can be improved; shown from the cumulative released amount of nitrogen of the biocharcoal-based slow release fertilizer in water, the cumulative released amount of the nitrogen in 480 hours is about 80%, i.e., the cumulative released amount of nitrogen nutrients in 20 days is 80%.The invention also discloses a preparation method of the biocharcoal-based slow release fertilizer. The preparation method adopts a hydrothermal method to prepare the biocharcoal-based slow release fertilizer, the technology is manure, the operation is convenient, simple and easy, low cost, less investment and low energy consumption are achieved, and an economic and feasible technical means is provided for large-scale application of the biocharcoal-based slow release fertilizer.