The invention provides a low-hardness brominated butyl rubber/polypropylene thermoplastic elastomer and a preparation method thereof. The low-hardness brominated butyl rubber/polypropylene thermoplastic elastomer is prepared from the compositions in parts by mass: 100 parts of brominated butyl rubber, 10-40 parts of ternary polymerization polypropylene, 5-100 parts of a styrene-isobutylene-styrenethermoplastic elastomer,0.1-5 parts of a vulcanizing agent, 0.1-5 parts of an acid absorbent and 5-60 parts of a plasticizer. The preparation method of the low-hardness brominated butyl rubber/polypropylene thermoplastic elastomer comprises the steps that the brominated butyl rubber, the styrene-isobutylene-styrene thermoplastic elastomer, the plasticizer, the ternary polymerization polypropyleneand the acid absorbent are first mixed uniformly in a high-temperature internal mixer, then the vulcanizing agent is added after the temperature is reduced, and rubber granulating is carried out; andfinally, granules are put into a twin-screw extruder for dynamic vulcanization to prepare the brominated butyl rubber/polypropylene thermoplastic elastomer. The brominated butyl rubber/polypropylenethermoplastic elastomer adopts the ternary polymerization polypropylene, the styrene-isobutylene-styrene thermoplastic elastomer is added, under the condition of the low plasticizer, low hardness andhigh air tightness are maintained, and meanwhile, TPV has a smooth granule appearance.