This study investigates the effects of different load characterization methods on the structural response of a 20-ton backhoe excavator working device under compound digging conditions. Based on field measured data, four typical compound digging trajectories were adopted, and the critical postures with peak cutting-loading resistance were selected for high-load analysis. A dynamic model of the working device was established via D’Alembert’s principle. Under identical postures and boundary conditions, three load schemes, namely measured excavation resistance, conventional compound digging force, and limit compound digging force, were applied to calculate structural loads on the boom and stick. Corresponding parametric finite element models were developed to compare structural equivalent stress and deformation. The results demonstrate that different load representations yield similar high-stress distributions and overall deformation modes for both components. The limit compound digging force produces higher maximum equivalent stress than the other two load schemes, providing a conservative evaluation for structural strength. Under the limiting load condition, the boom exhibits significantly increased deformation, while stick deformation presents non-monotonic variation due to the combined effects of load direction and working posture. This work can support design load selection and structural optimization for excavator working devices under compound digging operations.