The invention provides a Beta/ZSM-12 composite molecular sieve. The Beta/ZSM-12 composite molecular sieve has the following two crystal forms at the same time: a ZSM-12 molecular sieve and a Beta molecular sieve. The synthesis method for the Beta/ZSM-12 composite molecular sieve provided by the invention comprises the following steps: adding the ZSM-12 molecular sieve into a glucose solution for pretreatment so as to obtain a treated ZSM-12 molecular sieve; then mixing water, an alkali source, a template agent, an aluminum source and a silicon source and carrying out high-temperature crystallization so as to prepare pre-crystallized colloid; then mixing the pre-crystallized colloid with the treated ZSM-12 molecular sieve and carrying out crystallization at a temperature of 100 to 180 DEG C for 5 to 25 h so as to obtain the Beta/ZSM-12 composite molecular sieve. According to the invention, glucose is introduced into the pores of the ZSM-12 molecular sieve and becomes carbon after roasting; the carbon occupies the pore channels of the ZSM-12 molecular sieve to prevent alkali from entering the interior of ZSM-12 molecular sieve crystals, so the ZSM-12 molecular sieve is not destroyed in the process of crystallization and the integrity of crystal structure is maintained, which are beneficial for improving the physical and chemical properties and the catalytic activity of the Beta/ZSM-12 composite molecular sieve.