The invention relates to a cage-shaped flame-retardant charring agent trimethyl silicic acid PEPA ester compound and a preparation method of the cage-shaped flame-retardant charring agent trimethyl silicic acid PEPA ester compound. The structure of the compound is shown through the following formula in the specification. The preparation method includes the steps that at the temperature of 50 DEG C, trimethylchlorosilane is dripped to a solution of 1-epoxide phosphole -4-hydroxymethyl-2, 6, 7-trioxa-bicyclo [2.2.2] octane (PEPA) and an organic solvent, the temperature rises to 80 DEG C to 140 DEG C, a reaction is conducted for 5 h to 11 h through stirring, a quantitative acid-binding agent is added, stirring is conducted for 1 h, when the pH of the solution is detected as 5 to 6, the reaction is ended, after reduced pressure distillation, purification treatment is conducted, and then cage-shaped flame-retardant charring agent trimethyl silicic acid PEPA ester is acquired. The compound is an excellent halogen-free phosphorus and silicon synergistic flame-retardant charring agent, can be applied to polyolefin, polyester, polyvinyl chloride, polyurethane, epoxy resin, unsaturated polyester resin and the like, and is simple in process and small in equipment investment, and industrial production can be easily achieved.