The invention relates to a flame retardant tri(dichloropropyl)tribromophenylsilicate compound and a preparation method thereof. The structure of the compound is disclosed as the following formula, wherein n is a whole number ranging from 0 to 3. The preparation method comprises the following steps: dropwisely adding equal mole of epoxy chloropropane into a silicon tetrachloride organic solution at 20 DEG C below, adding equal mole of tribromophenol, and keeping the temperature at 60-80 DEG C to react for 2-4 hour; and after chlorine hydride is completely released, dropwisely adding a certain mole ratio of epoxy chloropropane, keeping the temperature at 90-120 DEG C to react for 6-9 hours, and distilling under reduced pressure to remove the solvent and excessive epoxy chloropropane, thereby obtaining the product tri(dichloropropyl)tribromophenylsilicate. The tri(dichloropropyl)tribromophenylsilicate is an excellent flame-retardant plasticizer, has the effects of charring and dripping prevention, has favorable compatibility with high polymer materials, and can be used as a flame retardant for polyvinyl chloride, unsaturated polyester, polyurethane, epoxy resin and other materials. The invention has the advantages of cheap and accessible raw materials, low production cost, low equipment investment and simple production technique, and can easily implement large-scale production.