The invention discloses a preparation method for heptamethyldisilazane, which comprises the following steps: (1) adding trimethyl chlorosilane and alkali metal compounds of monomethylamine and monomethylamino or monomethyl ethanolamine into a reaction kettle according to the molar mass ratio of 2:3.03-3.3, and performing high-shearing type reaction at the temperature of less than or equal to 120 DEG C and the pressure of less than or equal to 0.2 MPa; (2) when the reaction temperature approaches 120 DEG C, and the pressure approaches 0.2 MPa, maintaining the temperature and the pressure unchanged for more than 20 minutes, and ending the reaction; and (3) adding water into the reaction kettle, stirring the mixture, and standing and layering, wherein the upper layer is a product of the heptamethyldisilazane. The method provides a one-step synthesis method. The method has the advantages that: firstly, compared with the multi-step synthesis method, the method has simpler process, lower material cost, less equipment investment and lower production cost; secondly, the method adopts a non-solvent process method so as to improve the production capability and greatly reduce the energy consumption, and the product quality is better; and thirdly, water soluble salt technology is adopted, so the method simplifies the process, is easy to operate, and greatly improves the product yield.