The invention discloses a preparation method of a ZSM-5 (zeolites synthesized by mobil-5) molecular sieve catalyst with a small grain diameter. The preparation method comprises the steps of: dissolving silica sol into deionized water to mix uniformly to obtain a solution by a high-speed dispenser; dissolving NaOH into the deionized water and dripping into the a solution to mix uniformly to obtain b solution; taking Al2(SO4)3.18H2O to dissolve into the deionized water to mix uniformly, dripping the b solution to obtain c solution; taking H2SO4 to dissolve into the deionized water and dripping into the c solution to obtain white gel; adding ZSM-5 seed crystal; stirring to obtain silica-alumina gel; adding the obtained silica-alumina gel to a high-temperature reaction kettle, crystallizing, cooling to room temperature and washing; and exchanging by inorganic acid aqueous solution, and washing until the solution is neutral to obtain the ZSM-5 molecular sieve catalyst with the small grain diameter. By adopting the preparation method, the defects of preparation of the ZSM-5 molecular sieve catalyst with the small grain diameter by an organic template agent in a traditional technology are solved; not only is the stirring and aging time shortened, but also pollution of an organic matter can be removed, and energy consumption for high-temperature roasting is also reduced.