The present invention provides a method for preparing a mordenite, which is a method for preparing the mordenite through a hydrothermal crystallization by using natural aluminosilicate minerals as total silicon sources and total aluminum sources, comprising the steps of: subjecting the natural aluminosilicate minerals to activation treatment which is an activation by sub-molten salt and an activation by thermal treatment; selecting and mixing the activated natural aluminosilicate minerals according to the silicon-to-aluminum ratio of the targeted mordenite, adding thereto deionized water, sodium hydroxide, crystal seed, a templating agent to obtain a reaction mixture; adjusting the reaction mixture by using an inorganic acid to have a pH of 10-13, and then aging it at 40-80° C. for 6-24 hours; and subjecting the aged reaction mixture to hydrothermal crystallization, and then filtering, washing, drying and calcinating the crystallized product to obtain the mordenite. The method for preparing a mordenite provided in the present invention enlarges the sources of raw materials for a molecular sieve, has a low cost and a simple process route, and can largely reduce the cost for synthesizing a molecular sieve.