The present invention discloses a method of preparing a controlled-release nitrogen, phosphorus and potassium (NPK) compound fertilizer. The method comprises steps of preparing a PLA biopolymer at a predefined concentration between 5-15 %w/v by mixing the PLA biopolymer with chloroform; subjecting the biopolymer mixture to heating on a hot plate while stirring; forming a PLA film having a predefined thickness corresponding to the predefined concentration by casting the biopolymer solution into a mold and allowing the same to cool down to room temperature; retrieving the PLA film from the said mold; preparing a granular NPK compound fertilizer and positioning the same on the PLA film; encapsulating or wrapping the granular NPK compound fertilizer with the PLA film; and sealing the PLA film having the granular NPK compound fertilizer encapsulated or wrapped therewithin by way of hot-pressing.