SiGeSn alloys have high carrier mobility and long carrier lifetime, are compatible with complementary metal oxide semiconductors (CMOS), and possess unique optical properties and high thermodynamic stability. They are a type of semiconductor material with great potential for application in the field of optoelectronics. In this paper, eight models were constructed with a total atom count of 144, and Si/Sn atom ratios close to 3.7∶1. Their electronic structures and optical properties were calculated by the generalized gradient approximation (GGA+U) method within density functional theory. The results indicate that GeSiSn alloys with higher concentrations of Si and Sn have relatively higher binding energies and better stability. As the concentration of Si and Sn increases, the bandgap of GeSiSn materials gradually decreases. The imaginary part of the dielectric function peak shifts towards lower energy, the static dielectric constant decreases; changes occur in the absorption spectrum peaks within the visible and near-infrared regions; reflectivity increases in the ultraviolet and above wavelength range, refractive index decreases, extinction coefficient shifts towards the ultraviolet region, and photoconductivity extends towards the ultraviolet region. By adjusting the concentration while keeping the Si/Sn atom ratio fixed, suitable optical properties can be obtained. These results provide a reference for the application of GeSiSn alloy materials in optoelectronic materials and devices.