Cadmium sulfide (CdS) is the most widely used buffer material for a variety of thin-film solar cells including Cu2ZnSnS4. However, reports have shown that CdS film obtained by chemical bath deposition (CBD) is not the ideal buffer layer for pure sulfide CZTS solar cell. Zinc doping is a viable approach to modifying the CdS buffer film, but the present methods are far from satisfactory. Here, we innovatively developed an effective way of Zinc doping using partial electrolyte (PE) treatment, resulting in state-of-the-art buffer layer for solar cells based on high-surface-waviness CZTS light absorber. Our study shows that this Zn PE-treated CdS film improved the properties including coverage, full-range light transmittance and conduction band alignment with CZTS. Ultimately, the resultant modified CdS film boosted the open-circuit voltage of our devices by more than 100 mV, yielding power conversion efficiency (PCE) of 3.30%. We note that this is the highest efficiency that has been reported for all solution-processed CZTS solar cells.