Based on Darcy’s law for interdendritic flow, the present study developed a mathematical model for hydrogen porosity formation in Al castings, which specially considers the hydrogen macrosegregation including hydrogen diffusion and transport in macroscopic scale. An upward unidirectional solidification experiment of Al-4.5wt%Cu casting with in-situ melting was carried out. The microstructure analysis showed that the casting included columnar, CET (columnar-to-equiaxed transition), and equiaxed regions in upward direction. It was found that there exist a decreasing distribution of porosity in the columnar region and porosity with high content in casting part next to the bottom chill. The proposed model was applied to the experimental casting. The simulation results were in well agreement with the experiment results. It was indicated that the simulation without the hydrogen macrosegregation can’t well predict the microporosity in the casting, and that the hydrogen macrosegregation had significant influence on the predicted porosities, especially their nucleation and distribution.