Background:　The　COMPASSION-15　trial　demonstrated　that　cadonilimab　plus　chemotherapy　(CAD-CHM)　confers　clinical　benefits　over　placebo　plus　chemotherapy　(PLA-CHM)　as　a　first-line　treatment　for　human　epidermal　growth　factor　receptor　2　(HER2)-negative　advanced　gastric　or　gastroesophageal　junction　(G/GEJ)　adenocarcinoma.　However,　the　introduction　of　cadonilimab　substantially　elevates　treatment　costs,　and　its　cost-effectiveness　relative　to　PLA-CHM　remains　undetermined.　This　study　evaluates　the　cost-effectiveness　of　CAD-CHM　compared　with　PLA-CHM　from　the　perspective　of　the　Chinese　healthcare　system.　Methods:　A　Markov　model　with　three　health　states　was　developed　to　assess　the　cost-effectiveness　of　CAD-CHM　in　HER2-negative　advanced　G/GEJ　adenocarcinoma.　Clinical　efficacy　data　were　sourced　from　the　COMPASSION-15　trial,　while　drug　costs　were　calculated　based　on　national　tender　prices,　and　additional　costs　and　utility　values　were　extracted　from　published　literature.　The　analysis　encompassed　the　overall　population,　as　well　as　subgroups　stratified　by　programmed　death　ligand　1　(PD-L1)　combined　positive　score　(CPS)　＞=　5　and　CPS　＜　5.　Outcomes　included　total　costs,　quality-adjusted　life-years　(QALYs),　and　incremental　cost-effectiveness　ratios　(ICERs).　Sensitivity　analyses　were　conducted　to　evaluate　model　robustness.　Results:　The　ICER　of　CAD-CHM　was　$67,378.09　per　QALY　in　the　overall　population,　$48,433.34　per　QALY　in　the　PD-L1　CPS　＞=　5　subgroup,　and　$78,463.86　per　QALY　in　the　PD-L1　CPS　＜　5　subgroup.　Key　determinants　influencing　model　outcomes　included　patient　weight,　cadonilimab　cost,　and　the　utility　value　of　progression-free　survival.　Across　all　groups,　CAD-CHM　resulted　in　an　ICER　exceeding　the　willingness-to-pay　threshold　of　$41,511　per　QALY,　with　a　0%　probability　of　cost-effectiveness　compared　with　PLA-CHM.　Conclusion:　From　the　perspective　of　the　Chinese　healthcare　system,　CAD-CHM　is　not　cost-effective　as　a　first-line　treatment　for　HER2-negative　advanced　G/GEJ　adenocarcinoma,　either　in　the　overall　population　or　in　subgroups　stratified　by　PD-L1　CPS　status,　compared　with　chemotherapy　alone.