Cu2ZnSn(S,Se)4 (CZTSSe) thin films were prepared using the sol-gel method, and the crystal morphology of the CZTSSe films was improved by Mg doping. The prepared films were characterized using techniques such as X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy. The results showed that Mg replaced Zn in the CZTSSe lattice, forming the Cu2Zn1-xMgxSn(S,Se)4 (CMZTSSe) phase. As the Mg doping concentration increased, the grain size initially increased and then decreased. After Mg doping, no additional impurities are produced. When the Mg doping concentration was 0.1, the film exhibited the optimal crystal morphology, the narrowest peak width, the largest grain size, the best light absorption properties, the smoothest and most compact surface, which is favorable for use as an absorber layer in solar cells.