The invention discloses a core-shell type silicon-containing synergistic flame retardant and a preparation method thereof, which is characterized in that 50ml of deionized water and 150ml of ethanol are added into a reaction kettle equipped with a stirring and reflux device, and after stirring evenly, 50g of Ammonium polyphosphate and 1g OP-10, heated to 45°C under stirring, added concentrated ammonia water to adjust the pH value to 9-11, then slowly added 5g organic silicon oxide compound dropwise at a speed of 3ml / min, stirred for 150min, and then The reaction kettle was lowered to room temperature, filtered, washed, and the filter cake was dried at 120°C and 30mmHg. The average particle size of the obtained product does not increase significantly, the free flow of particles is good, the shell is dense and uniform, the water property is greatly reduced, and the high temperature water resistance is significantly improved. It can be used as a polymer material such as polyolefin, polyurethane, rubber, and foam. Flame retardant, it can also be used for flame retardant of paper, wood, fabric, etc.