Sapo-18 molecular sieve, belonging to AEI type structure, "pear-shaped" cage, is an eight-membered ring small-pore silicon-aluminophosphate molecular sieve with microporous structure composed of SiO2, ALO2- and P02+ three tetrahedral units. It has suitable acidity and high shape selectivity, and only has shape selectivity for c1-c4 hydrocarbons in the reaction process of methanol cracking to light olefins. Most of the products of methanol conversion are light olefins, and a very small amount of aromatic compounds and branched chain isomers are generated, which improves the methanol conversion rate and the yield of light olefins in the MTO reaction process.