The invention discloses a long-acting magnet therapy thermal underwear and a manufacturing method thereof. The long-acting magnet therapy thermal underwear is mainly formed by blending a foamy ferroferric oxide/graphene/cotton fiber compound and an unprocessed cotton fiber. The manufacturing method of the long-acting magnet therapy thermal underwear includes the following steps that the cotton fiber is preprocessed firstly and then is immersed into a solution containing Fe<3+>, Fe<2+> and oxidized graphene, then the cotton fiber containing the Fe<3+>, Fe<2+> and oxidized graphene is put in an alkaline solution, quick-freezing forming is performed after hydrothermal reaction, vacuum freeze-drying is performed to obtain the loose and porous foamy ferroferric oxide/graphene/cotton fiber compound containing an inorganic nano magnetic material, and the foamy ferroferric oxide/graphene/cotton fiber compound and the unprocessed cotton fiber are blended, and then the long-acting magnet therapy thermal underwear is manufactured according to conventional follow-up steps. The manufactured long-acting magnet therapy thermal underwear is good in warning effect, breathability and hygroscopic property, has magnet-therapy, antistatic and anti-microbial effects and the like, is easy to wash, is resistant to rubbing and is convenient to finish.