Based on the empirical electron theory of solids and molecules, the valence electron structures of S phase and the S/a interface were calculated, then the relationships between their VESs and precipitation strength and interface properties were analyzed in Al-Cu-Mg alloys in this paper. It is shown the covalent bond distribution of S phase is uniform and its main bond network is built by four stronger covalent bonds connected by Cu atoms. The nature of precipitation strength of S phase lies in its binding force of the strongest covalent bond is bigger 135.14% than that of the matrix a-Al and causes the stronger inhibition to the dislocation movement. The covalent electron density difference of (100)_S//(100)_a, (010)_S// _aand (001)_S//(021)_ais 0.003%, 3.564%and 5.811% respectively, while the covalent electron density of (100)_S//(100)_ais 10.3915 nm^_-2 and 10.3918 nm^_-2 , and that of (010)_S// _ais 0.0486 nm^_-2 and 0.0469 nm^_-2, and that of (001)_S//(021)_ais 0.0486 nm^_-2 and 0.0459 nm^_-2. Compared with (001)_S//(021)_a和(010)_S// _a, the binding force and covalent electron density of (100)_S//(100)_a is the biggest while its covalent electron density difference is the smallest, so that the interface continuity is the best and the interface stress is the smallest.