The invention provides a biochar-based organic special fertilizer for watermelons and a production method thereof. The special fertilizer is prepared from the following components in parts by weight:30-40 parts of biochar, 40-60 parts of organic materials, 6-8 parts of nitrogen (N2), 2-4 parts of phosphorus (P2O5), 6-9 parts of potassium (K2O), 10-15 parts of perlite, 1-2 parts of potassium sulfate, 1-2 parts of ammonium sulfate, 0.5-1 part of ammonium nitrate, 0.5-1 part of humic acid, 1.5-2.5 parts of wood vinegar, 1-2 parts of wood tar, 0.05-1 part of a probiotic agent, 0.5-1 part of a fertilizer controlled release agent, and 1-3 parts of a forming aid. The organic matters are solid livestock and poultry manure and decomposed organic plants with the water content of less than or equalto 40%. The preparation method comprises the following steps: carrying out high-temperature pyrolysis on crop straws to prepare biochar, adding the biochar and organic matters into a fermentation tank, carrying out mixed fermentation for 10-20 days, adding an ingredient granulator into the base material, granulating the mixture and finally spraying wood tar onto the granular fertilizer. The special fertilizer contains a large amount of organic nutrient elements, effectively utilizes waste resources, improves sandy soil, is suitable for planting watermelons, and improves taste and yield.