The invention relates to a method for preparing a mordenite molecular sieve. The method comprises the following steps of: dissolving an aluminum source in a sodium hydroxide solution, adding silicon sources, stirring at room temperature by strong magnetic force, and dispersing for a period of time; mixing the silicon sources which are dispersed uniformly and an aluminum source solution to form glue, adding a zeolite molecular sieve containing a five-membered ring as a crystal seed, stirring and mixing uniformly by the strong magnetic force at room temperature, and transferring the mixture into a reaction crystallization kettle, and performing crystallization reaction at the temperature of between 150 and 170 DEG C for 0.5 to 3 days; and performing suction filtration, washing and drying conventionally to obtain the mordenite molecular sieve. According to the method, an expensive organic amine template agent is not used as a structure guiding agent, and the zeolite molecular sieve which is wide in using, low in cost and readily available and contains the five-membered ring is used as the crystal seed to synthesize the mordenite molecular sieve with a high silica-alumina ratio directly, so the prepared mordenite molecular sieve has the advantages of high crystallinity and purity, controllable granular sizes and appearance, high yield, convenience for industrial production and the like.