A mononuclear cobalt complex [Co(H2L)(phen)2(H2O)]·4H2O (H2L2-=4,5⁃dihydroxy⁃1,3⁃disulfonate ion, phen=1,10⁃phenanthroline) was synthesized by hydrothermal method. The composition and structure of the complex were characterized by X ray single crystal diffraction, elemental analysis and thermogravimetric analysis. Single crystal diffraction study shows that the cobalt ion is six⁃coordinated, which can coordinate with the four nitrogen atoms of two phen ligands and the oxygen atoms of water molecule and sulfonate group, forming the twisted octahedral configuration in the complex. Compared with the ligand, the fluorescence emission peak of the complex has a blue shift, and the maximum emission peak is at 380 nm.