The invention discloses a preparation method of a ZSM-12 molecular sieve. The preparation method comprises the following steps: dissolving organic base, aluminum source and a structure-directing agent (R) shown in formula (I) in water and uniformly mixing the raw materials, wherein in the formula (I) which is defined in the description, n is equal to 4-6; adding a silicon source into the prepared mixed solution, uniformly mixing the raw materials to prepare a colloid or solid-liquid mixture, wherein the molar ratios of the reactants are that: SiO2/Al2O3 is 10-50, H2O/SiO2 is 5-50, R/SiO2 is 0.05-0.3, BA/SiO2 is 0.2-1.5, M2O/SiO2 is 0.02-0.3, the M2O is an alkali metal oxide, and BA is an organic base, the resulting colloid or solid-liquid mixture is subjected to first-stage hydrothermal crystallization reaction for 1-7 days at 80-140 DEG C under the self-generated pressure, and then heated to 150-250 DEG C and subjected to second-stage hydrothermal crystallization reaction for 5-14 days under the self-generated pressure, and a solid product is collected and dried. The ZSM-12 molecular sieve prepared by the method has a six-petal snowflake shape and has relatively good catalytic performance.