The invention relates to a conductive fiber and its preparation method. The conductive fiber has core-skin composite structure, the core layer is polyester and the cortical layer is polypropylene with containing conductive components, the skin-core ratio is 2:8-8:2; the quality percentage formula of cortical layer is: polypropylene 70-80%, nano-conductive powder 20 - 30%; said nano-conductive powder is tin oxide doped with antimony, the average diameter is not more than 0.1 micron, the resistance value is 10 omega.cm, its surface is treated and the treatment agents includes 3 - 10% coupling agent, 2 - 5% dispersing agent and 0.01- 0.1% anti-oxidant agent, said coupling agent is mono-alkoxylation catalyst barium ester or chloropropylated methoxyl silicon alky coupling agent; said dispersing agent is low molecular polyethylene wax; said anti-oxidant agent is four[ beta -(3',5'-di- tertiary-butyl- hydroxyl phenyl) ethylformic acid] pentaerythrite ester. The preparation method first prepares conductive polypropylene parent granule according said formula in the invention, then prepares conductive fiber through melting spinning technology.