Abstract: To reveal the intrinsic correlation between the phase stability mechanism and mechanical behavior of Cu–Sn intermetallic compounds (IMCs), the crystal structures, phase stability and elastic properties of typical phases including Cu₁₀Sn₃, Cu₃Sn, Cu₅Sn₄, Cu₆Sn₅, CuSn and CuSn₃ were systematically studied using the first-principles method based on density functional theory (DFT). The thermodynamic stability of each phase was determined through structure optimization and formation energy calculation. The single-crystal elastic constants were derived, and the average mechanical parameters of polycrystals were obtained via the Voigt–Reuss–Hill (VRH) model, followed by the analysis of elastic anisotropy. The results show that the CuSn phase has the lowest formation energy and the best thermodynamic stability. The Cu₃Sn phase exhibits the highest bulk modulus, indicating excellent compression resistance. Elastic anisotropy analysis reveals that CuSn and Cu₆Sn₅ have the largest and smallest universal anisotropy index. The directional dependence of elastic moduli differs significantly among these phases. This study provides theoretical support for the composition design and service performance optimization of high-reliability Cu–Sn-based electronic packaging materials.