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Cu₂Mg<i>_x</i>Zn_{1&#8722;<i>x</i>}SnS₄ (0 &#8804; <i>x</i> &#8804;0.6) thin films were prepared by a simple, low-temperature (300 &#176;C) and low-cost sol&#8722;gel spin coating method followed by post-annealing at optimum conditions. We optimized the annealing conditions and investigated the effect of Mg content on the crystalline quality, electrical and optical performances of the Cu₂Mg<i>_x</i>Zn_{1&#8722;<i>x</i>}SnS₄ thin films. It was found that the Cu₂Mg<i>_x</i>Zn_{1&#8722;<i>x</i>}SnS₄ film annealed at 580 &#176;C for 60 min contained large grain, less grain boundaries and high carrier concentration. Pure phase kesterite Cu₂Mg_<i>x</i>Zn_{1&#8722;<i>x</i>}SnS₄ (0 &#8804; <i>x</i> &#8804; 0.6) thin films were obtained by using optimal annealing conditions; notably, the smaller Zn²⁺ ions in the Cu₂ZnSnS₄ lattice were replaced by larger Mg²⁺ ions. With an increase in <i>x</i> from 0 to 0.6, the band gap energy of the films decreased from 1.43 to 1.29 eV. When the ratio of Mg/Mg + Zn is 0.2 (<i>x</i> = 0.2), the grain size of Cu₂Mg<i>_x</i>Zn_{1&#8722;<i>x</i>}SnS₄ reaches a maximum value of 1.5 &#956;m and the surface morphology is smooth and dense. Simultaneously, the electrical performance of Cu₂Mg<i>_x</i>Zn_{1&#8722;<i>x</i>}SnS₄ thin film is optimized at <i>x</i> = 0.2, the carrier concentration reaches a maximum value of 3.29 &#215; 10¹⁸ cm^&#8722;3.