Abstract: To accurately reveal the on-site expansive performance of weakly expansive soil and modified weakly expansive soil of the soil material yard of Huayang River Flood Storage and Detention Area Construction Project (Anqing city), in-situ water immersion expansion tests were conducted to study the changes in in-situ expansion deformation capacity of weakly expansive soil before and after modification. The results indicated that both cement mixing modification and overlying pressure could effectively inhibit the expansion deformation of weakly expansive soil. After excavation, mixing and backfilling, the expansion performance obtained from in-situ tests was significantly lower than the expansion properties of pure weakly expansive soil corresponding to the expansive grade determined by the survey, which might lead to overly conservative design schemes and resource wastage. It is recommended that the classification of weak expansiveness be based on half of the sample quantity. The research findings can provide support for the rational and comprehensive utilization of weakly expansive soil in similar embankment projects.