Flexible　Cu2ZnSn(S,Se)4　(CZTSSe)　solar　cells　show　great　potential　due　to　non-toxicity　and　low　cost.　The　quality　of　CZTSSe　absorber　suffering　from　the　high-temperature　selenization　process　is　the　key　to　overcoming　open-circuit　voltage　(VOC)　deficit　and　obtaining　high　efficiency.　In　this　work,　the　authors　develop　a　selenization　technique　to　improve　the　quality　of　the　CZTSSe　layer　by　pre-evaporation　selenization.　This　method　provides　saturated　selenium　vapor　at　the　beginning　of　the　selenization　process　to　promote　the　crystallization　process.　The　oversaturated　selenium　source　greatly　shortens　the　annealing　time　at　high　temperatures.　The　prepared　CZTSSe　films　have　larger　grains　and　fewer　voids.　The　measurement　of　the　space　charge　limited　current　shows　that　the　defect　density　of　the　CZTSSe　absorber　is　significantly　reduced　from　4.43 × 1013 cm−3　to　3.46　×　1013　cm−3.　Electrical　tests　show　that　the　devices　have　better　charge　separation　at　the　interface,　resulting　in　a　charge　recombination　lifetime,　increasing　from　133　to　197　µs.　The　power　conversion　efficiency　of　the　flexible　CZTSSe　solar　cell　reaches　10.24%　with　VOC　and　the　fill　factor　enhances　to　463 mV　and　62%,　respectively.　The　safe　non-toxicity　and　efficient　selenization　method　is　expected　to　provide　a　new　strategy　for　high-efficiency　flexible　CZTSSe　solar　cells.　©　2022　Wiley-VCH　GmbH.