Cerium conversion coating as an environmentally friendly conversion film can improve the corrosion resistance of materials. Cerium conversion coating was prepared on X80 by electrochemical deposition, and the corrosion behavior of the coating on X80 in 0.1 MPa and 20 MPa hydrostatic pressure 3.5% NaCl solution was studied. Open circuit potential, electrochemical impedance spectroscopy, linear polarization resistance and polarization curve were carried out by high temperature and high pressure online electrochemical testing autoclave. Scanning electron microscopy, energy disperse spectroscopy and contact angle meter were used to analyze the topography, elements and hydrophilicity of the surface before and after corrosion. Results show that the cerium conversion coating can significantly improve the electrochemical properties of X80 in shallow sea with a hydrostatic pressure of 0.1 MPa, while open circuit potential, linear polarization resistance and electrochemical impedance spectroscopy of cerium conversion coating on X80 obviously decline and corrosion current density increases in deep sea with a hydrostatic pressure of 20 MPa. Cerium conversion coating on X80 has much cracks after corrosion in deep sea with a hydrostatic pressure of 20 MPa, while no obvious defects is found in shallow sea with a hydrostatic pressure of 0.1 MPa. Cerium conversion coating shows hydrophilicity before corrosion, while contact angle increases after corrosion, showing hydrophobicity.