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The selenization annealing process is vital for highly efficient kesterite solar cells. Generally, SnS is added during the selenization process, but excessive S and related defects are introduced. Meanwhile, the path of supplementing Sn has never been elucidated. Herein, in order to solve the above problems, a combination of strategies involving SnS and Sn or SnSe or SnSe2 is put forward. And the composition of the vapor inhibiting Sn loss (gaseous SnSe3) and the pathway through which SnSe3 facilitates the formation of Cu2ZnSn(SxSe1-x)(4) (CZTSSe) are clarified. When SnSe2 is added to SnS in the selenization process, grain fusion is effectively promoted. The high crystalline quality kesterite absorber makes the band bending at the GBs optimal and the interface recombination be effectively suppressed. Moreover, cation disorder is remarkably reduced. Therefore, the open-circuit voltage (V-oc) is significantly elevated from 508 to 546 mV with increased fill factor (FF) and short-circuit current density (J(sc)). A state-of-the-art ambient air-processed kesterite device with 12.89% efficiency is achieved, and the unveiled reaction mechanisms have guiding significance for further optimizing selenization atmosphere and elevating the efficiency of CZTSSe solar cells.