Abstract: SAC305 solder alloy is one of the most widely used lead-free solder alloys, but insufficient reliability of SAC305/Cu solder joints limits its further popularization and application to a certain extent. In this study, AlCoFeNi_2.1 high-entropy alloy was used as a modifier and added to SAC305 lead-free solder with different mass fractions (0.0%, 0.1%, 0.2%, 0.5%, 1.0%, 2.0%). The effect of AlCoFeNi_2.1 high-entropy alloy on the properties of SAC305/Cu solder joints was systematically studied. The results show that after the AlCoFeNi_2.1 high-entropy alloy is incorporated into the solder, it is uniformly distributed in the SAC305 solder with fine irregular particles, which significantly refines the β-Sn phase grain and the grain size of the solder alloy. When the addition amount of high entropy alloy reaches 1.0%, the grain size reaches the minimum and there is no particle aggregation at the grain boundary. In addition, the addition of AlCoFeNi_2.1 increases a small amount of Co₃Fe₇ strengthening phase in the solder joint. The content of β-Sn phase decreases with the increase of the addition amount of high entropy alloy. The thickness of IMC layer decreases first and then increases with the increase of the addition amount, and the minimum is 4.11 μm when the addition amount is 1.0%. The spreading area of solder joints increases first, then decreases and then increases, and the wettability is the best when the addition amount is 0.2%. During the aging process, the thickness of the IMC layer increases with time, and the high-entropy alloy can effectively inhibit this trend. When the addition amount is 1.0%, the inhibition effect is the best, which significantly improves the long-term reliability of the solder joint, which provides a strong guarantee for improving the long-term reliability of the solder joint.