The optimum preparation condition of the polyaniline(PANI)/ manganese dioxide(MnO₂) composite cathode was confirmed by orthogonal design. The cathode was characterized by XRD and SEM. The electrochemical properties of PANI cathode were studied by electrochemical methods. The properties of sea-water battery combined by Mg alloy anode and PANI cathode were also investigated. The optimum condition of preparation is as follows: m(PANI)∶m(C₂H₄)=9∶1, using electronic grade electrolysis MnO₂ as stabilizer, and the forming pressure is 15 MPa. The specific energy of PANI cathode is up to 205.6 Wh/kg. The PANI particles is the spherical microstructure and have a higher crystallinity to some extent which is beneficial to improve the conductivity, decrease the polarization and promote the electrode reaction. The properties of sea-water battery are higher than those of AgCl-Mg alloy sea-water battery.