The invention relates to a charcoal based plant cultivation substrate and a preparation method thereof, and belongs to the technical field of soilless culture. The invention adopts the specific scheme that: after being dried, biomass material is pyrolyzed to form charcoal particles under an anoxic and heating condition, potassium dihydrogen phosphate is used for neutralizing the alkalinity of the charcoal particles and filtering and drying are performed, and after a certain amount of inorganic fertilizer is added, the charcoal particles are fully mixed with inorganic filling material, organic filling material and organic fertilizer, namely the charcoal based plant cultivation substrate is obtained. The invention has the advantages that the physical chemical property is excellent, the nutrients are complete, the fertilizer efficiency is lasting, the moisture adjusting capability is strong, the cost is low, the ecological benefits are high, the suitability is strong, the application range is wide, and the charcoal based plant cultivation substrate is suitable for cultivation of various plants.