The invention discloses a method for preparing an SAPO-11/MOR composite molecular sieve by using MOR (Mordenite) as a silicon source. The synthesis method comprises the following steps: performing alkali treatment and ion exchange on synthesized mordenite (MOR), uniformly mixing with a phosphorus source, an aluminum source and a template, adjusting the pH value by using ammonia water, and uniformly mixing so as to obtain a mixed solution; feeding the mixed solution into a polytetrafluoroethylene reaction kettle for crystallization; separating a solid product, washing, drying, and roasting to obtain the SAPO-11/MOR composite molecular sieve. The SAPO-11/MOR composite molecular sieve synthesized by using the method disclosed by the invention has a special morphology structure, SAPO-11 is subjected to in-situ growth on the surface of MOR in the presence of the silicon source in an MOR molecular sieve, the molecular sieve is small in distance and uniform in distribution, has proper acid distribution and relatively remarkable cooperativity, and isomerization reactions of alkane can be promoted.