Seismic　fragility　assessment　widely　uses　a　probabilistic　measure　for　assessing　the　seismic　performance　of　structural　components　or　systems.　This　article　proposes　a　copula-based　seismic　fragility　(CBSF)　method　to　derive　the　system-level　damage　probabilities　of　reinforced　concrete　bridges　and　to　consider　the　correlation　among　seismic　demands　of　components.　First,　the　marginal　distribution　functions　of　the　random　variables　are　calibrated,　and　three　Archimedean　copula　models　are　considered.　Subsequently,　the　relevant　parameters　of　the　copula　models　are　estimated　using　the　semi-parametric　maximum　likelihood　method.　Next,　the　damage　probabilities　of　a　bridge　system　are　calculated　based　on　the　joint　distribution　model　with　the　most　suitable　copula　model　and　the　calibrated　marginal　distribution　functions.　Finally,　the　CBSF　method　is　used　to　estimate　the　damage　probability　of　a　simply　supported　box　girder　bridge.　The　performance　of　the　CBSF　method　is　validated　by　a　comparison　of　fragility　curves　obtained　using　the　CBSF　method　and　the　probabilistic　seismic　demand　analysis　(PSDA)　method.　The　comparative　results　demonstrate　that　the　fragility　curves　obtained　by　the　CBSF　method　are　better　than　those　obtained　using　the　PSDA　method.　The　proposed　CBSF　model　can　serve　as　a　tool　for　assessing　the　seismic　performance　of　structures　and　estimating　the　economic　loss　of　existing　bridge　systems.