The invention relates to a method for preparing a nano zeolite molecular sieve by after-treatment of amorphous precursors. The method comprises the following steps: preparing amorphous aluminosilicate solutions with different silica-alumina ratios at room temperature, and stirring; sublimating in a freeze dryer to obtain amorphous gel solutions; centrifugating or settling and washing the gel solutions, and drying to obtain amorphous aluminosilicate precursors with different silica-alumina ratios; taking the amorphous aluminosilicate precursors, adding alkali liquor, and stirring uniformly; taking the amorphous aluminosilicate precursors, adding metal salts, stirring uniformly, treating at fixed temperature, adding alkali liquor, and stirring uniformly; and putting the solutions into a crystallization kettle, and carrying out two-stage heating to obtain the zeolite molecular sieve. Amorphous aluminosilicates with different silica-alumina ratios are used as precursors and subjected to after-treatment by different after-treatment solutions to controllably synthesize the zeolite molecular sieve nanocrystal with large external surface area and small crystal size; and the method has the advantages of simple reaction conditions, energy saving, environment friendliness and high yield.