Rechargeable magnesium-metal batteries (RMBs) have gained much attention due to their abundant resources as well as high safety. However, the high charge density of Mg²⁺ is one of the main reasons for the slow kinetics performance of RMBs, and modulation of the charge density is an important strategy to improve the kinetics and electrochemical performance of RMBs. Herein, we report on the conductive polymer polyaniline (PANI) for RMBs, which is found to have excellent kinetics and high discharge voltage when storing MgCl⁺. In the storage of MgCl⁺, PANI exhibits a high average discharge voltage platform is 2.3 V vs. Mg²⁺/Mg, which is higher than that in storage of Mg²⁺. We demonstrated the reversible intercalation/de-intercalation of MgCl⁺ in PANI accompanying with the reversible transformation between the quinone ring (C–C, –N=) and the benzene ring (C=C, –NH–) during charging and discharging. Density functional theory calculation reveals that PANI exhibit higher voltages (2.25 V vs. 1.82 V) along with lower diffusion energy barriers (1.27 eV vs. 1.55 eV) for MgCl⁺ storage compared to Mg²⁺ storage. This work refines the storage mechanism of PANI in RMBs and provides new guidelines for the application of PANI in RMBs.