The addition of Ca in magnesium melt has a significant influence on the heterogeneous nucleation effect of Al4C3 particles with open question for the mechanism. We have conducted first-principles calculations of the adsorption of Ca and Mg atoms on Al4C3 surfaces to study the initial nucleation and growth of Mg atoms on Al4C3 surfaces. The single Ca or Mg adsorption, as well as Ca-Mg co-adsorption on a 28-layer slab (A-type) of Al-terminated Al4C3(0001) surface has been simulated. We demonstrate that both Ca and Mg atoms tend to stabilize at the H2 site of Al-terminated Al4C3(0001) surface. Meanwhile, a comparative experiment is conducted to study the influence of Ca on the grain refining effect, showing that Ca atoms promotes the adsorption and nucleation growth of Mg atoms on Al4C3 surfaces.