This paper presents numerical studies of the melting process of stacked silicon with step porosity distribution, specifically its effect on seed crystal melting during quasi-single crystalline silicon casting for photovoltaic applications. The results show that an axial step porosity distribution in stacked silicon leads to a reduced melting ratio of the seed crystal. And the interface shape after seed crystal melting is mainly determined by the porosity at the lower part of the stacked silicon. In a specific range of the porosity, the axial step porosity distribution has little effect on the interface shape when the porosity difference between the upper and lower parts is small. For the situation with a similar melting ratio in the seed crystal, the interface shape of the seed crystal is flatter for a smaller average porosity. For the situation with a constant average porosity, a smaller porosity in the lower part of the stacked silicon is beneficial to the retention of the edge area of the seed crystal. The radial step porosity distribution dramatically influences the interface shape after seed crystal melting. As the porosity in the inner part of the stacked silicon increases, the interface shape changes from convex to concave after the seed crystal melting. To effectively retain the seed crystal, it is preferred to have the porosity in the outer part of the stacked silicon smaller than that of the inner part. And the size of the melting ratio of the seed crystal is directly related to the difference between the porosity at the outer and inner parts. The smaller the difference between the porosity at the outer and inner parts is, the smaller the melting ratio of seed crystals is. The melting ratio of seed crystal with axial step porosity distribution exhibits a certain symmetry, while that with radial step porosity distribution shows a certain periodicity. The interface shape after seed crystal melting is better under the uniform porosity distribution than others. Under the actual working conditions, the porosity distribution in the stacked silicon can be reasonably configured according to the contour map drawn by the melting state data of seed crystals obtained under different porosity distribution conditions. These findings provide guidelines for the optimal design of silicon stacking for the casting process.