The harsh operating environment in nuclear reactors imposes critical demands on structural materials. FeCoCrNi-based quinary high-entropy alloys (HEAs) integrate the benefits of Fe, Co, Cr, and Ni, while also incorporating elements like Al, Cu, and Ti to further enhance their properties. The content of these additional elements significantly influences the alloy's performance. This study employs molecular dynamics simulations to explore the microstructural changes and dislocation evolution in HEAs under tensile loading, considering factors such as alloy element type, content, and temperature. In this paper, tensile strains were applied to three HEAs—FeCoCrNiCux, FeCoCrNiAlx, and FeCoCrNiTix (hereinafter referred to as -Cux/Alx/Tix)—with x values such as 0.1, 0.4, 0.7, 1.0. Results showed that increasing x enhanced the mechanical properties of -Cux alloys but reduced those of -Alx and -Tix alloys. Additionally, the incorporation of Al and Ti had similar effects on the FeCoCrNi-based alloy system, with low concentrations (e.g., x ≤ 0.1) proving more beneficial than equal atomic fractions. The impact of Cu content within the studied range was not substantial, but the -Cu HEA with equal atomic fractions demonstrated superior mechanical properties. Optimal compositions (-Cu, -Al0.1, and -Ti0.1) were further subjected to tensile simulations across a temperature range from room temperature to extreme nuclear reactor conditions. It was observed that as the temperature increased, the mechanical properties of all three alloys decreased due to enhanced atomic thermal motion, which caused structural instability. Analyzing the mechanical properties at the atomic level is crucial for ensuring the safety and long-term stable operation of nuclear reactors, and it holds great significance for the research and development of new materials.