The invention discloses a preparation method of a sodium alga acid/ carbon nano tube composite fibre, comprising the following steps: completely predissolving and dissolving a sodium alga acid solution with the concentration of 2-7% by mass percent; mixing carbon nano tubes which account for 0.5-100% of the sodium alga acid solution by mass into the sodium alga acid solution, so as to obtain the sodium alga acid/ carbon nano tube spinning solution with good dispersibilty after complete mixing and ultrasonic dispersion; regulating the velocity of the spinning solution by sodium hypochlorite or hydrogen peroxide and the like, filtering and defoaming, and then carrying out solution spinning at room temperature to prepare the sodium alga acid/ carbon nano tube composite fibre, wherein the draw ratio of the fibre is 120-300%. The fibre produced by the invention realizes even dispersion and effective orientation of the carbon nano tubes in fibre and has higher draw tensile strength, the carbon nano tubes have extremely light ratio of quality to volume, thereby being applicable to military filed.