The present invention relates to a kind of preparation method of flame retardant methylphenylmethoxy (phosphacyclomethoxy) silane compound, the structure of the compound is shown in the following formula: the preparation method is: under the protection of nitrogen, add an organic solvent, Then add 1-oxygen-1-methyl-4-ethyl-4-hydroxymethyl-2, 6-dioxa-1-phosphorinane and methylphenyldimethoxysilane to control its The molar ratio is 1:1-1:1.4, the temperature is raised to 100-140°C, the temperature at the top of the fractionation column is controlled not to be higher than 65°C, and the generated methanol is fractionated, and the fractionation reaction is 6-10h until the fractionated methanol reaches the theoretical amount. The reaction is stopped, and after purification treatment, methylphenylmethoxyl (phosphacyclomethoxyl) silane is obtained; the product of the present invention has high flame retardancy, good compatibility with macromolecules, and is suitable for use as polyvinyl chloride, polyurethane, Flame retardant for polyester, unsaturated resin, epoxy resin, furan resin, polyvinyl alcohol and other materials. The preparation process is simple, the equipment investment is small, the large-scale production is easy, and the application and development prospects are very good.