According to the empirical electron theory of solids and molecules (EET) and Gerold’s terminal-state atom model of GP zone of Al-Cu alloy, the calculation model of valence electron structure of GP zone was set up and the evolution process was investigated. Moreover, the calculation method of lattice constant of GP zone during precipitation was proposed, and the valence electron structure of GP zone was calculated. Furthermore, the effect of Cu content on the valence electron structure was also studied. The results show that the formation of GP zone in Al-Cu alloy in early aging is a progressive process. In the initial development of GP zone, the Spinodal decomposition is the dominant mechanism, and in its later period, the dominant mechanism is the normal nucleation-growth mechanism. With aggregating of Cu atom, the covalent electron pairs nA on the strongest covalent bond A increase gradually, but the covalent electron pairs nB, nC and nD on the covalent bonds B, C and D decrease bit by bit. The strengthening of A bond in GP zone is by means of the transfer to A bond of covalent electrons on B, C and D bonds