The invention discloses a preparation method of a nano-hybrid flame retardant. The preparation method comprises steps as follows: water-soluble divalent metal salt and water-soluble trivalent metal salt are dissolved in deionized water, and a first solution is prepared, wherein a molar ratio of the divalent metal salt to the trivalent metal salt is (1:1)-(8:1); water-soluble hypophosphite is dissolved in deionized water, and a second solution is prepared; the first solution and the second solution are dropwise added to a reaction vessel simultaneously and are mixed uniformly, a mixed solution is formed, and pH of the mixed solution is regulated by a strong alkaline solution to 7.5-11; the mixed solution is heated, a crystal compound is formed and is subjected to water washing, filtering and drying processing, and the nano-hybrid halogen-free flame retardant is obtained. The invention further relates to the nano-hybrid flame retardant acquired with the method.