The invention provides a synthesis method of an IM-5 molecular sieve. The method comprises the steps of: (1) under a stirring condition, contacting an aluminum source-containing mixed solution with a silicon source to make a colloid, with the aluminum source-containing mixed solution being an aqueous solution containing inorganic alkali, a mineralizer, an aluminum source and a template agent; and (2) under crystallization reaction conditions, subjecting the colloid to hydrothermal crystallization. According to the composition of the colloid, the mole ratio of the inorganic alkali, the mineralizer, the aluminum source, the template agent, the silicon source and water satisfies the following formula: SiO2:Al2O3:M2O:MX:SDA:H2O=10:(0.1-1):(1-10):(0.1-5):(0.1-4):(50-500). The silicon source is SiO2, the aluminum source is Al2O3, the inorganic alkali is M2O, MX is the mineralizer, and SDA is the template agent. The mineralizer is a metal halide able to regulate the molecular sieve crystal growth. The method can be used for preparing the IM-5 molecular sieve with enhanced crystallinity.