Abstract: The stability of deep excavated expansive soil canal slopes during operation is influenced by multiple factors including groundwater levels, primary fissures, and wetting-drying cycles. The deformation and failure mechanisms under their synergistic effects remain unclear. To address this, a model of an expansive soil slope containing primary fissures was constructed, capable of simulating groundwater level fluctuations Four wetting-drying cycles and one prolonged rainfall event were simulated, with groundwater level changes controlled during the rainfall process. The development of surface fissures and internal and external deformations were observed during the test. The results indicate: (1) Surface fissures exhibit spatially differentiated evolution. The surface fissure ratio and average fissure width at the slope crest gradually increase with cycles, while those at the slope middle and toe decrease due to groundwater and lateral constraints; (2) Primary fissures significantly control slope deformation. The maximum surface settlement at the fissure-side slope middle reaches 37 mm, and deep soil layers also undergo significant deformation influenced by the fissure plane. In contrast, deformation on the non-fissure side primarily occurs within the shallow soil layer at depths of less than 10 cm; (3) During the fourth drying stage, under the influence of a high groundwater level, the coupling effect of fissures and seepage leads to a sharp increase in deformation, triggering failures such as rill erosion, toe uplift, and shallow landslides. Primary fissures not only provide preferential seepage channels for groundwater but also, together with groundwater level fluctuations, weaken the soil and amplify deformation responses. This study reveals the synergistic evolution laws of fissure-seepage-deformation under multi-field interactions, providing theoretical support and engineering guidance for the stability assessment and protective reinforcement design of expansive soil slopes.