The invention relates to a lamellar mordenite preparation method, which comprises: (1) dissolving an inorganic alkali, an aluminum source and a structure-directing agent (R) in water and uniformly mixing to prepare a solution, wherein the structure-directing agent is a 1,n-bis(N-methylpyrrolidine)alkylene bromine salt having the structure formula represented by a formula (I), and n is 2-5; (2) adding a silicon source to the solution prepared in the step (1), and uniformly mixing to prepare a colloid or solid-liquid mixture, wherein the molar ratios of the added reactants comprise that the SiO2/Al2O3 is 10-50, the H2O/SiO2 is 5-50, the R/SiO2 is 0.05-0.5, and the M2O/SiO2 is 0.05-1.0, and the M2O is an alkali metal oxide; and (3) carrying out hydrothermal crystallization for 1-5 days at a temperature of 80-140 DEG C under autogenous pressure, heating to a temperature of 150-250 DEG C, carrying out hydrothermal crystallization for 6-14 days under autogenous pressure, collecting the solid product after the crystallization, washing, and drying, wherein the obtained product has characteristics of high crystallinity, small grain thickness, and simple synthesis method. The formula (I) is defined in the specification.