<p>The confirmed benefits of tandem blades, i.e., low flow loss coefficient and wide working range, make them more competitive than conventional blades when applied in the rear stator of a multistage axial compressor with excessive flow turning from high swirl to axial exhaust. However, the three-dimensional design space of tandem blades remains underexplored so far, and the complex relationships between aerodynamic performance and geometric parameters are not yet fully clarified. To address this gap, the present work employs the Shapley additive explanations (SHAP) method combined with the extreme gradient boosting (XGBoost) metamodel to conduct data mining on three-dimensional tandem-bladed stators in the rear stage of a gas turbine compressor in the present work. The results show that the characteristics of forward sweep and positive bow, combined with increased gap-nozzle dimension at 25% span and reduced dimension at hub side, are effective in enhancing the aerodynamic performance of the examined rear-stage compressor. The newly designed tandem-bladed stator achieves reduced total pressure losses by 1.59%–5.63% at typical operating points, without compromising static pressure recovery. Flow analyses indicate that the “C-shaped” spanwise pressure redistribution along the suction surface of the aft blade and the reduced blade loading near the hub, are the primary contributors to these aerodynamic improvements. The present work supports three-dimensional design of advanced tandem blades for axial-flow compressors.</p>