•Hot extrusion greatly modifies the microstructure of AP65 alloy.•Hot extrusion effectively enhances the discharge performance of AP65 alloy.•The correlation between microstructure and performance of AP65 alloy is analysed. AP65 is a promising magnesium alloy used as anode for high-power seawater activated battery. This alloy is specially required to exhibit a high anodic efficiency and provide a negative discharge potential with a short activation time for the potential to reach the steady value. Here we demonstrate that this is possible, using a hot extrusion at 450°C with an extrusion ratio of 24.5:1. The discharge behaviour of AP65 alloys before and after hot extrusion is investigated by electrochemical techniques. Furthermore, the microstructure evolution during hot extrusion and its impact on the discharge performance of AP65 alloy are also analysed.