Annealing CZTS precursors in Se-containing atmosphere to obtain large grain CZTSSe absorbers has proven to be a promising method for preparing high-efficiency solar cells. However, due to Se diffusion from surface to inside of the precursor film, inhomogeneous CZTSSe grain growth can be generally observed in the traditional selenization process. The undesired fine-grain layer forms beneath the top large-grain layer and harmful secondary ZnSe phase generates on CZTSSe/Mo interface on account of the insufficient selenization, thereby degrading the device performances. To derive high-quality CZTSSe absorbers, a novel two-step annealing process was performed. CZTS precursors were firstly annealed at a low temperature and then annealed at a high temperature. Compared with the traditional one-step process, there is a remarkable promotion of average device efficiency from 3.86% to 9.58% using the two-step process. The mechanism for improving device performance was proposed: The formation of densely packed large-grain layers on surface of the absorber was delayed under the low-temperature annealing stage, the Se spread to the inside of the absorber from the grain boundaries, then providing driving force for grain growth and suppressing the decomposition of CZTSSe in subsequent annealing process. Finally, CZTSSe solar cell with the highest efficiency of 10.19% was achieved based on the two-step process.