The　traditional　CdS　buffer　layers　in　flexible　CZTSSe　solar　cells　lead　to　light　absorption　losses　and　environmental　pollution　problems.　Therefore,　the　study　of　Cd-free　buffer　layer　is　very　important　for　the　realization　of　environmentally　friendly　and　efficient　CZTSSe　solar　cells.　The　Zn1-xMgxO　(ZnMgO)　and　Zn1-xSnxO　(ZnSnO)　alternate　buffer　layers　are　studied　in　this　study　using　the　simulation　package　solar　cell　capacitance　simulator　(SCAPS-1D)　numerical　simulation　model,　and　the　theoretical　analysis　is　further　verified　by　the　results　of　the　experiments.　We　simulate　the　performance　of　CZTSSe/ZnXO　(X　=　Mg/Sn)　heterojunction　devices　with　different　Mg/(Zn+Mg)　and　Sn/(Zn+Sn)　ratios　and　analyze　the　intrinsic　mechanism　of　the　effect　of　conduction　band　offsets　(CBO)　on　the　device　performance.　The　simulation　results　show　that　the　CZTSSe/ZnXO　(X　=　Mg/Sn)　devices　achieve　optimal　performance　with　a　small　“spike”　band　or　“flat”　band　at　Mg　and　Sn　doping　concentrations　of　0.1　and　0.2,　respectively.　To　investigate　the　potential　of　Zn0.9Mg0.1O　and　Zn0.8Sn0.2O　as　alternative　buffer　layers,　carrier　concentrations　and　thicknesses　are　analyzed.　The　simulation　demonstrates　that　the　Zn0.9Mg0.1O　device　with　low　carrier　concentration　has　a　high　resistivity,　serious　carrier　recombination,　and　a　greater　impact　on　performance　from　thickness　variation.　Numerical　simulations　and　experimental　results　show　the　potential　of　the　ZnSnO　buffer　layer　as　an　alternative　to　toxic　CdS,　and　the　ZnMgO　layer　has　the　limitation　as　a　substitute　buffer　layer.　This　paper　provides　the　theoretical　basis　and　experimental　proof　for　further　searching　for　a　suitable　flexible　CZTSSe　Cd-free　buffer　layer.　©　2023　Chinese　Physical　Society　and　IOP　Publishing　Ltd.