The invention discloses a method for reducing the pore diameter of a small crystal grain ZSM-5 (zeolite socony mobil-5) molecular sieve. The method includes: making an aluminum source, an additive (A) and a template agent (T) into solution, adding a silicon source after well mixing to enable the molar ratio of components to satisfy SiO2:Al2O3:T:A:H2O=(40-500):1:(10-80)(>0-10):(100-2000), well mixing at the room temperature, crystallizing for 6-72 hours at 100-200 DEG C, cooling, centrifugally separating and washing after crystallizing, and drying to obtain the product. The small crystal grain ZSM-5 molecular sieve which is controllable and reduced in pore diameter can be directly synthesized after the additive (A) is added, the treatment step of modification for reducing the pore diameter of the ZSM-5 molecular sieve is omitted, the catalyst preparation process is shortened, shape selectivity of the ZSM-5 molecular sieve is improved, and the catalysis performance of the ZSM-5 molecular sieve used in the fields of methanol conversion for preparing propylene, benzene alkylation and the like is optimized.