The structures of liquid IU-Si alloys modified by Sr were measured by using a high temperature x-ray diffractometer. The structure factor and the coordination numbers of liquid Al-Si alloys with different Si content have been obtained by Radial distribution functions, the nearest atomic distance and the coordination numbers of eutectic AL-SI alloys were studied, before and after being modified with Sr. The radial distribution functions of the liquid alloys were decomposed by 5 Gaussian peaks. These results show that Si-Si covalent bond exists in the liquid eutectic and hypereutectic alloys. Sr in the liquid AI-Si has a capability to weaken the covalent bonds of Si-Si, suppressing the nucleation of eutectic silicon phase.