Abstract: Using phosphogypsum to improve expansive soil can simultaneously achieve the resource utilization of phosphogypsum and the improvement of expansive soil′s swelling characteristics.Taking the expansive soil from section Y003 of the Yangtze-to-Huaihe River Water Diversion Project as an example, we investigated the engineering properties evolution law of phosphogypsum-modified expansive soil through liquid limit, plastic limit, compaction, swelling rate, direct shear, and disintegration tests.The results showed that: the liquid limit and plasticity index of the modified soil were lower than those of the untreated expansive soil.When the phosphogypsum content was 50% (mass percentage), the liquid limit decreased by 19.7 percentage points, and the plasticity index decreased by 20.3 compared to the untreated soil.The swelling rate of the modified soil was consistently lower than that of the untreated soil.When the phosphogypsum content was 50%, the free swelling rate decreased by 36 percentage points and the load-free swelling rate dropped by 5.12 percentage points.As the phosphogypsum content increased, the internal friction angle of the modified soil gradually rose, while the cohesion gradually declined.The addition of phosphogypsum reduced the soil′s resistance to disintegration, so other cementitious materials was suggested to add in practical applications.This research provides a reference for similar projects that use modified expansive soil.