Abstract: Concrete-faced rockfill dams (CFRDs) rely on key impermeable structures, such as concrete slabs, for their safe operation. Ensuring the safety of the concrete slab is of utmost importance. The current evaluation method, which utilizes uniaxial strength, overlooks the influence of slab stress on its strength and may lead to unreasonable judgments regarding the damage zone. To address this issue, this study introduces a method for evaluating the stress in concrete slabs based on a multi-axial strength criterion. The safety of the slab is assessed by determining the concrete strength adjustment coefficient and the multi-axial strength stress level index. A refined stress distribution analysis of the slab is conducted using a cross-scale fine simulation method based on the multi-point constraint. The improved viscoelastic method is employed to comprehensively simulate the impact of dynamic and permanent deformations on the slab stress. This approach accurately depicts the progression of slab damage during an earthquake. To illustrate the effectiveness of the proposed multi-axial strength method, a 240 m-high concrete-faced rockfill dam located in a narrow valley within a seismic zone is taken as an example for comparison with the current uniaxial strength method. The results demonstrate that, during the operational period, the material strength on both sides of the dam abutment is reduced, while it is enhanced on the riverbed, particularly at the bottom. Both methods yield similar conclusions regarding the tensile damage zones, but there are significant disparities in the compressive damage zones. During an earthquake, the multi-axial strength method indicates slightly smaller tensile damage zones, whereas the location and extent of the compressive damage zones differ significantly between the two methods. Considering the limitations of the uniaxial strength method in considering the influence of slab stress on its strength and the potential for misjudgment of the danger zone, this study suggests adopting the multi-axial strength criterion as a guide for designing and constructing concrete slabs in extra-high CFRDs.