To　improve　the　energy　efficiency　and　the　data　rate　in　differential　chaos　shift　keying　(DCSK)　based　ambient　backscatter　communication　(AmBC)　system,　we　propose　a　new　AmBC　system　based　on　code　index　modulation　(CIM),　referred　to　as　CIM-DCSKAmBC　system.　In　the　proposed　system,　the　CIM-DCSK　signal　transmitted　in　the　direct　link　is　used　as　the　radio　frequency　source　of　the　backscatter　link.　The　signal　format　in　the　backscatter　link　is　designed　to　increase　the　data　rate　as　well　as　eliminate　the　interference　of　the　direct　link　signal.　As　such,　the　direct　link　signal　and　the　backscatter　link　signal　can　be　received　and　demodulated　simultaneously.　Moreover,　we　derive　and　validate　the　analytical　bit　error　rate　(BER)　expressions　of　the　CIM-DCSK-AmBC　system　over　cascaded　multipath　Rayleigh　fading　channels.　Regarding　the　short　reference　DCSK-based　AmBC　(SR-DCSK-AmBC)　system　as　a　benchmark　system,　numerical　results　reveal　that　the　CIMDCSK-AmBC　system　can　achieve　better　BER　performance　in　the　direct　link　and　higher　throughput　in　the　backscatter　link　than　the　benchmark　system.　IEEE