The invention relates to an L/Silicalite-1 compound molecular sieve and a preparation method thereof. The compound molecular sieve has an L zeolite crystal phase and a Silicalite-1 molecular sieve crystal phase. The preparation method of the compound molecular sieve comprises the steps of: adding L zeolite powder in a solution containing sodium hydroxide and tetrapropylammonium bromide or tetrapropyl ammonium hydroxide, adding a silicon source, uniformly stirring to obtain a reaction mixture gel system, wherein the mol ratio of the mixture of the reaction mixture gel system is 4.5-6.0 Na2O:35-10 SiO2:12-2.0 (TPA)2O:Al2O3:200-290H2O; and crystallizing the reaction mixture at a temperature of 120-180 DEG C for 8-24h under a closed condition to obtain the L/Silicalite-1 compound molecular sieve. The compound molecular sieve provided by the invention is a novel product, and can be used in multiple catalysis processes such as a process of producing olefins by the synthesis gas, and the like.