The invention discloses a preparation method for modifying ethyl alcohol to prepare olefin by adopting a molecular sieve to catalyze. The preparation method is characterized in that a TEOS is taken as a silicon source, and aluminum nitrate is taken as an aluminum source; and a novel modified H type Sapo-5/MCM-41 composite molecular sieve is synthesized by magnetizing and oil-bath heating. The composite molecular sieve is used for catalyzing alcohol to prepare olefin, so that an ethyl alcohol conversion rate is very high, and ethylene selectivity is very high.