<p>The soil water retention curve (SWRC) is a crucial indicator in the analysis of percolation. For low plasticity soil containing substances that promote soil expansion and contraction, the volume change behavior of the soil is generally neglected to determine the drying SWRC. Such a procedure is constrained by an underlying assumption that the volume change of the soil is zero or negligible, which consequently limits the precision of seepage simulations. In this study, for the exogenous substance of soil swelling and shrinking, Attapulgite (ATP), the concept of “effective porosity” was introduced to develop a modified SWRC model that takes into account the variation of soil pore space based on the conventional van-Genuchten (VG) model of simulated SWRC. The parameters of the modified SWRC model were determined using a genetic algorithm, which was used to simulate infiltration in the SWMS_2D program to further evaluate the accuracy of the model. The results indicated that the modified model provides a more precise representation of the relationship between soil water content and suction, with error analysis presented outstanding goodness-of-fit. As a critical factor controlling the modeling of soil–water interaction, this modified SWRC model parameters were found to be in good agreement with simulation results of the seepage process of soil water flow over the soil column. The evaluation conducted via the Taylor diagram proved that the model is more precise than the conventional VG model in simulating the seepage process. In particular, the greater the soil volume change caused by exogenous ATP, the superior the performance of the model.</p>