The invention belongs to the technical field of intelligent materials, relating to an electrochromism composite fiber. The composite fiber comprises a hollow carbon nano tube; polydiacetylene is formed in the carbon nano tube, when current is not led into the two ends of the composite fiber, the composite fiber presents the first color; when the current led into the composite fiber exceeds a critical value, the color of the composite fiber changes into the second color; and when the current led into the two ends of the composite fiber is turned off, the color restores into the first color. The preparation method of the composite fiber comprises the following steps: preparing a carbon nano tube erect column by a chemical vapor deposition method, and spinning into the carbon nano tube fiber; and dipping diyne monomer to the carbon nano tube fiber, and irradiating by utilizing ultraviolet rays, X-ray and gamma-ray to polymerize the diyne monomer into the carbon nano tube/polydiacetylene compound fiber. The electrochromic effect of the compound has repeat reversible comprehensive electrochromic performance, and has wide application potency and simple preparation method.