NOVELTY - Sustained-release bio-fertilizer is prepared by taking 1-10% potassium fertilizer, adding 0.1-1.5% cross-linking agent bound with 0.2-0.3% binder, then wrapping using coating material, drying the coated mixture, and granulating the dried mixture into granule of 3-5 mm in diameter, where the mixture is wrapped with 20-50% coating material and 0.2-1% potassium solubilizing bacterial culture liquid, and the weight ratio of organic and inorganic components are 5-20:95-80.    USE - The sustained-release bio-fertilizer is useful reducing insect pest and diseases in plant.    ADVANTAGE - The sustained-release bio-fertilizer reduces waste of potassium, improves microflora environment in plant rhizosphere, and soil nutrient balance and fertility, and absorbs potassium.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method for preparing sustained-release bio-fertilizer, which involves (a) taking soil sample, diluting soil at different concentrations for culturing, performing streak plate method for purification, inoculating 5-10% seed bacterial solution in a medium containing potassium feldspar powder, increasing the quantity of potassium in the medium to determine the ability of each bacterial solution using flame photometer, and expanding potassium solubilizing bacteria, where potassium solubilizing bacteria is cultured at 25-35 degrees C for 36-60 hours with pH of 7.1-7.9 to obtain 0.2-1% potassium solubilizing bacterial culture liquid, (b) dissolving binder, and cross-linking agent in water to obtain 1-5% binder, and cross-linking agent solution, (c) uniformly mixing organic and inorganic components coated materials to obtain coating material, (d) rotating potassium in a rotary granulator, spraying suitable quantity of binder, continuously spraying binder and cross-linking agent after the formation of granular potassium, again adding potassium fertilizer, repeating process, adding coating material to granule of 2-3 mm in diameter, continuously spraying binder and cross-linking agent to the granule, and spraying solution of potassium fermentation liquid, and repeating process to obtain semi-finished product of 3-5 mm in diameter, and (e) drying the semi-finished product in oven at 35-45 degrees C for 10-24 hours.