Heavy paint coating loss and localized corrosion of the Al alloy hull occur red after less than a year’s service of a ship at sea.Examination and analysis showed that the Mg-basic sacrificial anodes originally used for cathodic protection were not suitable as the resulting -1.3V-plus (vs Ag/AgCl) protection potential for the hull was too negative for the purpose .A new protection system was designed in which the protection current densities were 15mA/m2 for the hull and 350mA/m2 for the propeller - The hull protection potential under actual condition at sea was between -920mV and - 1100mV (Ag/AgCl) and the protection efficiency was over 90% . The protection areas were calculated by using the formulas A=1.8×LBP×D+ BC× LB×B for the hull and As=2Aas for the propeller. Heavy paint coating loss and localized corrosion of the Al alloy hull occurred red after less than a year’s service of a ship at sea. Examination and analysis showed that the Mg-basic sacrificial anodes originally used for cathodic protection were not suitable as the resulting -1.3 V-plus (vs Ag / AgCl) protection potential for the hull was too negative for the purpose. A new protection system was designed in which the protection current densities were 15 mA / m2 for the hull and 350 mA / m2 for the propeller - The hull The protection areas were calculated by using the formulas A = 1.8 × LBP × D + BC × LB × B for the hull and As = 2Aas for the propeller.