The CZTSSe/CdS interface qualities are vital aspects and factors that would greatly decide the photovoltaic performance of CZTSSe solar cells. High-quality CZTSSe/CdS interfaces could reduce defect density and improve CZTSSe device performance. However, due to the inhomogeneous CZTSSe grain growth in local micro-regions during selenization, void defects would emerge at the CZTSSe/CdS interfaces by traditional CdS deposition method. In order to obtain high-quality CZTSSe/CdS interfaces, a novel method of two-step CdS deposition, with which the two CdS layers were deposited before and after the selenization of CZTS absorbers, was firstly introduced in the work. The first CdS layer with a certain designed thickness was deposited onto the as-deposited CZTS thin films by CBD. Then the thin films were annealed in selenium-containing atmosphere to obtain CZTSSe thin films of large grain sizes. The second CdS layers with different thicknesses were deposited onto the annealed CZTSSe thin films by CBD again. Comparing with that by the traditional one-step CdS deposition method, the efficiencies of CZTSSe solar cells by two-step CdS deposition method show a remarkable increase from 7.25% to 10.19%. An action mechanism for improving device performance by two-step CdS deposition was proposed: The first CdS layer on CZTS precursors could consistently keep a well coverage for the CZTSSe grains on the surfaces during selenization; The second CdS layer could ensure p-n junctions in good condition, which would reduce recombination centers and shunting paths. The results suggest that the two-step CdS deposition process is an effective method to obtain high-quality CZTSSe/CdS interfaces and high-performance solar cell devices.