The soil with application of water retention agent was taken as an object to establish a model of porosity change, and the parameters of the model were calculated and verified by measuring the porosity of the soil under different moisture conditions. The model was found to have better simulation effects. On this basis, water-swelling property and porosity change of the soil with application of water retention agent were analyzed. The results show that the more water retention agent is applied, the more the soil swells and the more the porosity increases.