The invention relates to the technical field of thermoplastic elastomers, in particular to a halogen-free flame retardant thermoplastic elastomer and a preparation method thereof. The halogen-free flame retardant thermoplastic elastomer comprises, by weight, 75-110 parts of polyurethane elastomers, 12-30 parts of phosphorus based flame retardant, 5-35 parts of phosphorus-nitrogen containing flame retardant, 1-3 parts of inorganic salt flame retardant, 5-12 parts of plasticizer and 1.3-3 parts of lubricant. The halogen-free flame retardant thermoplastic elastomer solves the problem that an existing elastomer containing halogen flame retardant can generate toxic and corrosive gas, and the smoke formation quantity can be greatly controlled in the open fire burning process of the elastomer. According to the halogen-free flame retardant thermoplastic elastomer, the above raw materials are adopted, the weight ratio of all the raw materials is strictly controlled, the flame retardant effect is good, the flame retardant can reach the UL94-V0 level, fireproofing is good, and the smoke formation quantity is small; precipitation is low, dripping can be prevented, high electrical performance can be achieved, and the heat resistance is good. Moreover, the abrasion resistance is excellent, ozone resistance is extremely good, hardness is large, strength is high, elasticity is good, and comprehensive performance is excellent.