The invention relates to a method for preparing chloromethyl methyl dimethoxysilane. The current method has the problems of long cycle time of the reaction process, harsh conditions, and complicated reprocessing, and so on. In the invention, the method comprises the following steps: firstly, injecting the chloromethyl methyl dimethoxysilane into a storage tank; secondly, mixing methyl ortho formate and methanol in a reaction kettle, and adding the chloromethyl methyl dimethoxysilane into the reaction kettle at a temperature of between 40 and 90 DEG C for 0.5 to 4 hours; obtaining the chloromethyl methyl dimethoxysilane after atmospheric distillation; and collecting gas of methyl chloride and methyl formate released in the reaction process by a cryotrap. The method of the invention has theadvantages of mild reaction condition, high yield, short reaction process, and easily operated feeding and post treatment, thereby easily realizing large scale industrial production.