The atomic bonding of the segregated cell of Al-Li alloy in earlier ageing condition was calculated according to the empirical electron theory (EET) in solid. The results showed that the strongest covalent bond was the Al-Al bond in the segregated cell without vacancy, while the strongest covalent bond was the Al-Li bond in the cell containing vacancy. Since the difference of electronagativity between the Al and Li atoms was obvious, it was inclined to form the Al-Li segregated cell of short range order structure in the condition of vacancy joining. The short range order structure with vacancy was probably the embryo or precursor structure of the metastable phase ' δ (Al3Li). As the ' δ (Al3Li) was coherence with matrix, the bond net strength was enhanced by the precipitation of ' δ (Al3Li) and hence the alloy was strengthened.