NOVELTY - A biological compound fertilizer preparation method involves collecting microbial sample, storing in a sterile plastic bag and preparing a culture medium. A screening culture medium and a fermentation culture medium are provided with low-grade rare earth ore powder, nitrogen and potassium, followed by concentrating a leaching solution to provide potassium, sulfur, phosphorus, trace elements and rare earth elements in liquid biological compound fertilizer, where the liquid biological compound fertilizer is dried and granulated to obtain solid biological compound fertilizer.    USE - Method for preparing a biological compound fertilizer.    ADVANTAGE - The method enables preparing the biological compound fertilizer with reduced pollution and reduced energy consumption in a simple and cost-effective manner.    DETAILED DESCRIPTION - A biological compound fertilizer preparation method involves collecting microbial sample from rare earth mine collecting soil sample and storing in a sterile plastic bag at 4 degrees C. A culture medium is prepared with target microorganism growth. A screening culture medium and a fermentation culture medium are provided with low-grade rare earth ore powder, nitrogen and potassium. Decomposition low grade rare earth ore of the target microorganism is selected by weighing soil sample in sterile water and coating an enriched culture medium flat plate at 26-30 degrees C for 3-4 days. A strain is inoculated to the fermentation culture medium at 30 degrees C for 20 days to obtain the target microorganism to prepare composite microbial inoculum. Solid fermentation is performed by crushing the low grade rare earth ore, mixing 2 pts. wt. fermentation material, 2-3 pts. wt. lees, 5 pts. wt. waste molasses and 2-3 pts. wt. bean cake powder, adding water, mixing, adding complex microbial inoculum, mixing uniformly and stacking to ferment for 15-25 days to obtain the solid fermented product. Biological leaching solution is prepared by putting the solid fermented product in a biological leaching device, adding sterile water, adding the composite microbial inoculum, collecting extract and leaching for 25-35 days. The leaching solution is concentrated to provide potassium, sulfur, phosphorus, trace elements and rare earth elements in liquid biological compound fertilizer. The liquid biological compound fertilizer is dried and granulated in a leaching device to obtain solid biological compound fertilizer.