The present invention relates to a fucoidan-based theragnostic composition, and more particularly, to preparation and use of a theragnostic composition which uses a conjugate of a fluorescent dye or a photosensitizer and fucoidan, thereby not only allowing for fluorescence imaging diagnosis of lesions for cancer or vascular diseases but also allowing a therapeutic effect thereon to be obtained at the same time. The conjugate obtained by covalent bonding of a fluorescent dye or a photosensitizer and fucoidan, according to the present invention, is not only useful for fluorescence imaging diagnosis of tumor tissues and ophthalmic vascular diseases, but also may exhibit a therapeutic effect on cancer cells and coronary artery smooth muscle cells, and a neovascularization inhibitory effect in ophthalmic diseases. In addition, in a case where photodynamic therapy is further implemented on the conjugate according to the present invention, cancer and vascular diseases may be effectively treated with low adverse effects.
