The invention discloses a nitrogen-phosphorus-silicon ternary halogen-free synergistic flame-retardant thermoplastic elastomer and a preparation method thereof. The thermoplastic elastomer specifically comprises, by weight, 2-20 parts of phosphorus-nitrogen-containing tetrafunctional acrylate, 10-45 parts of a styrene block copolymer, 20-60 parts of filling oil, 5-20 parts of polypropylene, 5-10 parts of a maleic anhydride grafted styrene block copolymer, 0.1-2 parts of a stabilizer, 20-40 parts of thermoplastic polyurethane, 10-30 parts of an organic silicon and polyurethane segmented copolymer, and 1-5 parts of a silicone master batch. The preparation method comprises the following steps: weighing the raw materials in proportion, then mixing the raw materials in a high-speed stirrer, then putting the mixture into a double-screw extruder, performing melt extrusion, and after extrusion, cooling, drying and pelletizing to obtain the finished product. The thermoplastic elastomer with excellent flame retardance is prepared by adopting nitrogen-phosphorus-silicon ternary halogen-free synergistic flame retardance, and the mechanical property and smoothness of the material are not sacrificed while the flame retardance of the material is greatly improved. The flame-retardant thermoplastic elastomer is suitable for being applied to the fields of automobile parts, sealing strips, wires and cables, medical apparatuses and instruments and the like.