Modified fiber and a preparation method therefor. By means of a physical method, uniform dispersion of graphite particles in a polyester base material and a polyamide base material is realized, the technology is simple, a dispersing agent is not needed, and industrial production is easy. By introducing graphene into hollow cotton or polyamide fiber, and particularly introducing biomass graphene into the hollow cotton or polyamide fiber, modified hollow cotton or polyamide fiber is enabled to have a low-temperature far infrared function, the far infrared normal emittance thereof being not less than 0.85; the antibacterial performance is greater than 90%; and the heat preservation performance and the air permeability are both remarkable, and when the content of the biomass graphene is 1.4%, the heat preservation rate is equal to that of white duck down and is about 90%, but the air permeability is about 240 mm/s and is far higher than that of the duck down.