The invention relates to a flame retardant charring agent-tetra-{1-oxyphospha-2,6,7-trioxabicyclo-[2.2.2]-octane-(4)-methyl} silicate compound and a preparation method thereof. The compound has a structural formula shown in the specification. The preparation method comprises the following steps of dropwise adding silicon tetrachloride into a solution of 1-oxyphospha-4-hydroxymethyl-2,6,7-trioxabicyclo-[2.2.2]-octane (PEPA) and an organic solvent at 20 DEG C, heating to 80-140 DEG C, performing stirring reaction for 4-10 hours, adding a fixed quantity of an acid-binding agent, stirring for 1 hours, and ending the reaction when detecting that the pH value of the solution is 5-6; cooling, performing suction filtration, washing by water, and drying to obtain tetra-{1-oxyphospha-2,6,7-trioxabicyclo-[2.2.2]-octane-(4)-methyl} silicate. The compound is the excellent halogen-free phosphorus-silicon synergistic flame retardant charring agent and can be applied to polyolefin, polyester, polyvinyl chloride, polyurethane, epoxy resin, unsaturated resin and the like, the process is simple, the equipment investment is low, and the industrialized production is easy to realize.