Although trace silver (Ag) substitution for Cu in Cu₂ZnSn(S, Se)₄ (CZTSSe) offers benefits in improving open-circuit voltage (V_OC) of CZTSSe solar cells, its further development is still hindered by the fairly low short-circuit current density (J_SC) owing to the increase of bandgap (E_g) of the absorber layer. Herein, a strategy is demonstrated to mitigate the poor J_SC arising from the Ag substitution by tuning the Cu²⁺/(Cu⁺+Cu²⁺) percentage in the (Cu, Ag)₂ZnSnS₄ (CAZTS) precursor solution. Depth characterization indicates that the appropriate Cu²⁺/(Cu⁺+Cu²⁺) percentage in the CAZTS precursor solution increases the photogenerated charges separation and carrier transport processes for Ag-substituted CZTSSe (CAZTSSe) solar cells. Benefiting from this strategy, the power conversion efficiency (PCE) of 9.57% is achieved when the Cu²⁺/(Cu⁺+Cu²⁺) is 50 at.%. The main factor for the improvement of PCE is due to the increase in photogenerated current density (J_L) and decrease in shunt resistance (R_sh). It is demonstrated that the increase in J_L results from a widening in the depletion layer width (W_d) of the p-n junction and an increase in carrier lifetime, induced by optimization of the Cu²⁺/(Cu⁺+Cu²⁺) percentage. While the decrease in R_sh comes from the change of reaction pathways due to the change in Cu²⁺/(Cu⁺+Cu²⁺) percentage.