Alkaline-earth metal-based MOF has many merits such as light weight, environmental friendliness and low cost, but it has rarely been used in energy storage and capacitive deionization (CDI). This paper presents a facile method for the fast synthesis of small-size magnesium formate MOF (Mg-MOF) under mild conditions by linker-free incubation in the presence of Mg(CH3COO)(2). 4H(2)O and dimethylformamide. The formic acid was in situ formed as ligand to facilitate the formation of Mg-MOF. The effect of incubation time on the morphology of MOF crystals was investigated. The small and well-defined Mg-MOF particles with the size of similar to 18 nm were formed in 1 h incubation, characterized by scanning electron microscopy and X-ray diffraction. Following carbonization, the Mg-MOF-derived mesoporous carbon was obtained. N-2-sorption isotherm confirmed the mesoporous structure. Cyclic voltammetry test indicated the good electric double-layer feature of the Mg-MOF-derived carbon electrode, and the impedance results showed the excellent conductivity. As a capacitor electrode material, it delivered the specific capacitances of 102 F/g by GCD (0.2 A/g). The Mg-MOF-derived carbon electrode was applied successfully for capacitive deionization (CDI) with a desalination capacity of 8.0 mg/g in a CDI device. A supper fast desalination rate of 1.1 mg/g/min was achieved due to the abundant mesoporous structure. The work provides a cost effective and environmentally friendly way for the synthesis of MOF-based carbon materials free of transition metal, and the prospects for CDI application have been demonstrated.