The invention relates to a preparation method of an initiator for synthesizing a high polymer, in particular to a preparation method of emulsion type tert-butyl peroxyneo-caprate (BNP). According to the technical scheme of the invention, the preparation method of the emulsion type BNP comprises the following steps of: mixing octyl decanoyl chloride with an alkaline solution and tert-butyl hydrogen peroxide, stirring and reacting at the temperature of 10-40 DEG C; separating a mother liquor; and mixing a reaction product with a homogenized liquid prepared from an anti-freezing agent, an emulsifier, a dispersant and deionized water to obtain the emulsion type BNP. Due to the adoption of the method provided by the invention, the product content and yield are increased greatly, and convenience is brought to storage and transportation. Due to the adoption of the method, the process flow is simplified, the complex requirements on reaction equipment are eliminated, a reaction can be undergone under the mild and safe condition, and the product yield can be over 95 percent.