We have performed molecular dynamics simulations on a typical Al-based alloy Al90Sm10. The short-range and medium-range correlations of the system are reliably produced by ab initio calculations, whereas the long-range correlations are obtained with the assistance of a semi-empirical potential well-fitted to ab initio data. Our calculations show that a prepeak in the structure factor of this system emerges well above the melting temperature, and the intensity of the prepeak increases with increasing undercooling of the liquid. These results are in agreement with x-ray diffraction experiments. The interplay between the short-range order of the system originating from the large affinity between Al and Sm atoms, and the intrinsic repulsion between Sm atoms gives rise to a stronger correlation in the second peak than the first peak in the Sm-Sm partial pair correlation function (PPCF), which in turn produces the prepeak in the structure factor.