In recent years, the primary reasons for low efficiency Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have been attributed to SnZn defects and related defect clusters, as well as the balance of absorption layer bandgap for short-circuit current density (Jsc) and open-circuit voltage (Voc). Ge gradient has been theoretically proven to be an effective strategy to change traditional flat bandgap structure and suppress SnZn defects and related defect clusters for improving the device performance of CZTSSe solar cells. However, the potential of Ge gradient has not been fully verified. In this work, Ge doping effectively reduces the formation of SnZn defects and [2CuZn + SnZn] defect clusters in the CZTSSe absorption layer. In addition, it can be found that the construction of the V-type bandgap is the best solution for balancing between Voc and Jsc. By using the bandgap grading strategy, the disparity between the recombination barrier of the junction and the optical bandgap's minimum value is widened, thereby obtaining a large Voc. The V-type doped device has the highest efficiency when Voc is 0.45 V, which can reach 8.03%. This Ge graded substitution method provides an alternative absorption layer structure for improving Voc and the preparation of future high-efficiency kesterite photovoltaic devices.