Zn(O,S)　is　a　promising　alternative　buffer　layer　to　CdS　in　Cu2ZnSnS4　(CZTS)　based　solar　cell　due　to　its　large　bandgap　and　nontoxic　elements.　In　this　study　the　performance　of　CZTS/Zn(O,S)/Al:ZnO　solar　cell　was　numerically　simulated　by　Solar　Cell　Capacitance　Simulator　(SCAPS).　And　the　sulfur　content　of　Zn(O,S)　buffer　layer　was　varied　to　investigate　how　the　chemical　composition　of　Zn(O,S)　influences　the　conduction　band　offsets　at　absorber/buffer　and　buffer/window　interfaces　and　then　affects　the　cell　performance.　It　was　found　that　the　conduction-band　offset　(CBO)　of　the　CZTS/Zn(O,S)　heterojunction　played　a　significant　role　in　the　performance　of　the　solar　cell.　The　electron　affinity　and　bandgap　of　Zn(O,S)　are　controlled　by　the　sulfur-to-oxygen　ratios,　and　the　resulting　offset　span　is　from　+0.7　eV　in　the　＇spike＇　direction　to　-0.1　eV　in　the　＇cliff＇　direction　if　the　electron　affinity　of　CZTS　is　considered　as　4.5　eV.　When　S/(S+O)　atomic　ratio　of　Zn(O,S)　is　about　0.3,　the　cell　has　a　high　conversion　efficiency　of　14.90%　with　CBO　of　0.2　eV.　The　simulation　results　will　provide　some　important　guidelines　for　fabricating　high　efficient　CZTS　solar　cells.