AlCoCrFeMo0.05Ni2 high-entropy alloy (HEA) has great potential for high-temperature applications. However, its FCC matrix will decompose during annealing, which seriously limits its high-temperature applications. Here, the lattice distortion, Gibbs free energy change, phonon spectra, density of states, and bond length distribution of the FCC matrix were calculated by first-principles calculations. The findings indicate that the primary barrier to the formation of a single homogeneous solid solution in the alloy is more closely associated with Gibbs free energy rather than the degree of lattice distortion. The vibration frequency of Al is not in sync with the overall structure, making it prone to desolvation at elevated temperatures. Furthermore, an analysis of bond lengths reveals that the Al–Ni bond length is significantly shorter than the theoretical value, facilitating the formation of a (Ni, Al)-rich phase. In contrast, the longer bond lengths of Al–Cr and Al–Mo facilitate the detachment of Cr and Mo from the (Ni, Al)-rich phase, resulting in the formation of (Cr, Mo)-rich phase. This in-depth investigation sheds light on the phase decomposition mechanism of high-entropy alloys, offering valuable insights for future research in this field.