Metallic 1T MoS2 greatly benefits the supercapacitance of MoS2 due to its considerably higher conductivity as compared to semiconducting 2H MoS2. Alkali metals, such as Li, Na and K, are always used to prepare 1T MoS2 through intercalation of alkali metal ions into the interlayer of 2H MoS2. Nevertheless, the influences of the alkali-earth metals as the guest in the interlayer of MoS2 on its structure and electrochemical capacitor performance are rarely investigated. Herein, we introduced hydrated Mg ions as the guest into MoS2 nanosheets. The interlayer spacing was increased to 1.144nm after the introduction of hydrated Mg ions, larger than that of the pristine MoS2 (0.620nm) and restacking MoS2 (0.626nm). The enlarged interlayer spacing can accommodate more ions during intercalation process. Moreover, the 1T phase concentration after the introduction of hydrated Mg ions was as high as similar to 90%, which benefits the charge transfer during the charging/discharging processes. Consequently, the specific capacitance of the MoS2 with Mg guest ions as well as the energy densities and power densities was greatly improved as compared to those of the restacking MoS2 or pristine MoS2 counterparts. The present work not only demonstrated a new strategy for engineering the physical properties and improving the electrochemical performance of MoS2 supercapacitor electrode through pre-intercalation of alkali-earth metal ions in the interlayer of MoS2, but also has directive significance for the design of other layered electrode materials for energy storage systems.