The invention relates to ZSM-5 molecular sieve with small crystal grains and application thereof, mainly solving the technical problems that the silica-alumina molar ratio (SiO2/Al2O3) of ZSM-5 molecular sieves with small crystal grains is less than 200, and the traditional ZSM-5 molecular sieves have poor catalyst stability and short service life when used for reaction for preparing propylene from methanol in the prior art. The invention well solves the problems by the technical scheme that the ZSM-5 molecular sieve with small crystal grains, which has a crystal diameter less than 300 microns and a SiO2/Al2O3 molar ratio not less than 200, is used for reaction for preparing the propylene from the methanol. The invention can be used in industrial production for preparing the propylene from the methanol.