The valence electron structure of GP Zone in Al-Zn alloy was calculated according to the Empirical Electron Theory(EET). The result of valence electron structure was further applied to analyze the interface energy between GP Zone and matrix. The reason that the GP Zones were formed even at the highest quenching speed and the hardness was stably ascended as soon as GP zones were formed and reached the maximum before the precipitation of metastable phase, was that the amoumt of the strongest covalent bond in the GP Zone was far more than that in alpha-Al cell and the Al atom in GP Zone was easily prone to forming covalent bond with Zn atom. It was an easy and effective method to apply the EET theory with the hardball model to calculation of interface energy.