Abstract: The interaction mechanism between silane coupling agent and nano silicon dioxide in the process of in-situ preparation of nano silicon dioxide composite material was studied by sol-gel method. The effects of silane coupling agent dosages on the mechanism and the application performance of the nano silicon dioxide composite material in epoxy varnish were also investigated. The mechanism of preparing nano silicon dioxide composite material is that the organic group of silane coupling agent substitutes the physisorption water and silicon alcoholic group on the surface of the nano silicon dioxide and forms nano composite material with good dispersibility. When the dosage of silane coupling agent is suitable, the composite material exhibits a good application performance in epoxy varnish as well as reinforcing and toughening properties of nano materials. The preparation technology has a good application prospect.