The invention relates to a method for preparing an SAPO-11/MOR composite molecular sieve with preset MOR crystal seeds. The method comprises the following synthetic steps: after synthesized mordenite MOR undergoes alkali treatment and ion exchange, a hydrogen type molecular sieve is prepared; deionized water is uniformly mixed with a phosphorus source, an aluminium source, a template and a silicon source, so that a mixed solution A is obtained; the treated MOR is added into the solution and uniformly mixed, so that a mixed solution B is obtained; the mixed solution B is loaded into a polytetrafluoroethylene reaction kettle, and is put into a crystallization reaction oven for crystallization; a solid product is separated, washed, dried and roasted, and thereby the SAPO-11/MOR composite molecular sieve is obtained. The composite molecular sieve synthesized by the method disclosed by the present invention shows a package type structure with SAPO-11 as a shell and MOR as a core, and the chemical interaction between SAPO-11 and MOR enables the composite molecular sieve to have appropriate B acid distribution.