Impressed current cathodic protection (ICCP) is an effective corrosion protection and prevention technique for reinforced concrete structures served in chloride-contaminated environment. The acidification damage in the external anode mortar caused by the anodic reactions significantly influences the efficiency of ICCP for reinforced concrete. Therefore, improving the acidification resistance of the external anode mortar is of great importance for the successful application of ICCP treatment. In this study, a novel type of lightweight functional aggregates was proposed to mitigate the acidification damage in the external anode mortar. The influence of the prepared lightweight functional aggregates both on the electrochemical performance of the primary anode and morphology, mineral compositions and microstructure of the secondary mortar were investigated. The results indicated that the lightweight aggregates efficiently mitigated the damage of MMO coating on the primary anode surface, thus maintaining the high stability and catalytic activity of the primary anode. Severe acidification damage happened in the mortar matrix within a distance of 300 μm from the primary anode; beyond this region, the acidification damage was mainly propagated along the interfacial transition zone around the aggregates. Because the lightweight functional aggregates maintained the high alkalinity in the secondary mortar, the acidification damage both in the vicinity of the primary anode and ITZ around aggregates was significantly halted. Therefore, the prepared lightweight functional aggregates can be potentially used for preparing the high performance external anode mortar, further improving the stability and efficiency of ICCP treatment and durability of reinforced concrete structures.