The invention discloses a composite modified SAPO-34 molecular sieve and a preparation method thereof. The preparation method comprises: (1) preparing a SAPO-34 molecular sieve; (2) modifying the SAPO-34 molecular sieve by using a metal; and (3) modifying the SAPO-34 molecular sieve by using a persulfate, wherein the metal modified SAPO-34 molecular sieve is impregnated into a persulfate solutionaccording to a volume ratio of 1:1 to prepare the composite modified SAPO-34 molecular sieve. According to the present invention, the catalyst can promote and achieve the conversion process of low-temperature methanol-to-low-carbon-olefin through the SAPO-34 molecular sieve, improve the selectivity of the SAPO-34 molecular sieve to the low-carbon olefin at the low reaction temperature, and enhancethe low-temperature activity stability of the molecular sieve catalyst.