Different sizes of layered CoOOH were synthesized by the molten-salt-assisted method at different temperatures. X-ray diffraction and scanning electron microscope studies reveal that CoOOH grew at (003) with increasing temperature, and its size can reach dozens of microns. X-ray absorption near edge structure and XPS studies demonstrate that the Co valence state of CoOOH-750 is trivalent, and X-ray Absorption Fine Structure shows that it had a higher symmetry and lower disorder degree, indicating that CoOOH-750 has higher crystallinity and Co3+. The results of electrochemical tests show that CoOOH-750 exhibited the best oxygen-evolution-reaction (OER) catalytic activity.