In the present work, kesterite Cu2ZnSn(S,Se)(4) (CZTSSe) films were successfully prepared by tuning the cationic ratios (Zn/Sn and Cu/(Zn +Sn)) with DMF-based solution approach and the effect of cationic ratios on the performance of solar cells with the CZTSSe as absorbers was studied. Our results revealed that the interfacial secondary phases and interfacial structure of CZTSSe/Mo can be optimized by changing the cationic ratios so as to decrease the reverse saturation current (J(0)) and increase the shunt resistance (R-sh). And the photocurrent (J(L)) can be improved by broadened depletion region layer (W-d) of the solar cell due to decreased net hole concentration (N-B) induced by decreased Cu/(Zn + Sn). By optimizing the cationic ratios, the CZTSSe solar cell with the highest power conversion efficiency (PCE) of 8.01% was obtained at Cu/(Zn + Sn) = 0.75 +/- 0.05 and Zn/Sn = 1.18 +/- 0.02. The increase of PCE is mainly attributed to the decrease of interfacial secondary phases and the improvement of interfacial structure.