The invention relates to a method for preparing a molecular sieve, and is aimed at providing a method for preparing a nano flake FER molecular sieve by a hydrothermal synthesis method. The method comprises the following steps of uniformly mixing water and an aluminum source, and adding a N, N-diethyl-2,6-diemethyl-piperidinium solution and solid NaOH, adding a silicon source while stirring, and continuously stirring the mixed solution until the mixed solution is gelatinous; transferring the mixed solution to a reaction kettle to perform a dynamic crystallization reaction for 36-96 hours at thetemperature of 110-150 DEG C; filtering and drying the reaction product to obtain the FER molecular sieve raw powder. According to the invention, the FER molecular sieve with the thickness of about 7nm synthesized by the hydrothermal single template method is utilized, so that the thickness of the FER molecular sieve is greatly reduced, the synthesis is convenient, and the energy is saved. Compared with the prior art, only one organic template agent is used in the whole production process, and the prepared product maintains good crystallinity and purity and has good catalytic reaction activity. Therefore, the method has great significance in the field of practical chemical production.