NOVELTY - A complex fiber material with a core-shell structure, where the core part and the shell part are separately an oriented is obtained by in situ growing netty nitrogen-doped carbon nano-tube on the surface of carbon nano-tube fiber. The complex fiber is flexible and knittable.    USE - For preparing oxygen reduction and hydrogen peroxide detection electrode (claimed).    ADVANTAGE - The complex fiber material provides excellent mechanical and electrical performance; reduces oxygen reduction current density; and using hydrogen peroxide detects micro electrode with high sensitivity and low cost.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a preparation method of a complex fiber material, involving: (1) firstly synthesizing a spinnable carbon nano-tube array with a chemical vapor deposition (CVD) method, taking nano-Fe/Al2O3 as a catalyst, ethylene as a carbon source, and mixed gas of argon and hydrogen as a gaseous phase carrier, carrying out chemical vapor deposition for obtaining the spinnable carbon nano-tube array; consequently, spinning from the carbon nano-tube array for obtaining a carbon nano-tube fiber; (2) then soaking the carbon nano-tube fiber into ferric chloride (FeCl3) aqueous solution (0.1-0.2 M), evaporating the water, absorbing the FeCl3 in the carbon nano-tube fiber; (3) consequently, placing the carbon nano-tube fiber containing FeCl3 into a CVD tubular furnace, under a mixed atmosphere of argon and hydrogen, raising the temperature from room temperature to 890-910 degrees C within 20-25 minutes, and keeping stable for 40-60 minutes at 890-910 degrees C for reducing FeCl3 into Fe; (4) finally, switching the carrier gas into a bottle containing ethanediamine, introducing the ethanediamine serving as a carbon source and a nitrogen source into the CVD tubular furnace by the carrier gas for growing a nitrogen-doped carbon nano-tube for 5-30 minutes for obtaining nitrogen-doped complex fiber materials with core-shell structures and with different contents.