Seepage-induced softening often leads to shallow instability in canal slopes composed of expansive soil. Overburden replacement with cement-modified soil has emerged as a commonly employed method to address this issue. This study focuses on the weak expansive soil along the route of the Yangtze-to-Huaihe Water Diversion Project. Using the GDS triaxial apparatus, we conducted triaxial weak expansion tests on the soil with an initial moisture content of 20% and a compactness of 96%. By examining the variations of volumetric strain and final moisture content with average principal stress, we established a triaxial expansion model that accounts for water content increment and discussed the model’s potential engineering applications. Furthermore, we analyzed the stability of the canal slope and proposed treatment measures. Results indicate that the non-uniform expansion deformation within a certain depth range caused by the shallow expansion of canal slope is the primary factor contributing to slope instability. For low channel slope sections comprised of weak expansive soil (H≤8.7 m), a compressive load of 0.3 m ensures project safety during the operation. The research finding offers a theoretical and design foundation for optimizing engineering design and reducing project costs.