The invention relates to a preparation method of 1-methyl-2-propylimidazole, and especially relates to a method for synthesizing 1-methyl-2-propylimidazole under negative pressure with a gas-liquid-liquid phase method with pure methylamine, ammonia gas, pure butyraldehyde and a glyoxal water solution as raw materials. According to the method, pure butyraldehyde and the glyoxal water solution are added into a gas-liquid-liquid phase synthesis device; under a controlled temperature and a controlled negative pressure, ammonia and pure methylamine are delivered in at a same time; after the addition, the temperature and the negative pressure are continuously controlled, and a reaction is allowed for 24h; after the reaction, water is removed by distillation; and cooling, crystallization, filtering and drying are carried out, such that a product is obtained. With prior arts, the pressure of a reaction system is higher than environmental pressure, such that leakage of gases such as methylamine can be easily caused. With the method provided by the invention, the problem is solved.