The invention relates to a mono-nuclei cationized magnesium salt, a preparation method and applications thereof. The mono-nuclei cationized magnesium salt has a chemical formula of MgRnMX4-mYm, wherein R is a non-aqueous solvent molecule, M includes Al3+ and/or B3+, X and Y respectively include halide ion and halogenoid ion, n is any one integer selected in the range of 0˜6, and m is any one integer selected in the range of 0˜4. The mono-nuclei cationized magnesium salt provided by the invention has a simple structure and excellent electrochemical properties, and the preparation method thereof features low cost, integrated synthesis, accessible raw materials, simple preparation process, and simple scaled production. The provided mono-nuclei cationized magnesium salt is used as the electrolyte of the rechargeable batteries, the generated electrolyte solution has a high ionic conductivity, a high reversible magnesium deposition-dissolution efficiency, excellent circulating performance and a high anode oxidation deposition potential. For example, when the electrolyte solution is applied to the magnesium batteries, the initial discharging capacity of the batteries can reach over 700 mAh/g, and the cycle number is greater than 20.