Located in a back-arc sedimentary basin in Gandise island-arc, the Jiama copper and polymetallic deposit occurs as stratiform and stratoid bodies in the transitional zone between the reef limestone (J-3d) and sandy slate (K-1l). Stratabound skarn is the ore-bearing rock in the Jiama deposit. With its unique geological characteristics, it has aroused great interest among investigators. Based on REE and silicon isotope characteristics of the Jiama stratabound skarn and a comparison with typical magmatic-hydrothermal contact metasomatism skarn, Sedex deposit and modern seafloor hydrothermal fluid, the authors hold that the stratabound skarn in Jiama deposit is different greatly from the typical magmatic-hydrothermal contact metasomatic skarn, but is comparable with modern seafloor hydrothermal fluid, Sedex deposit and associated hydrothermal sedimentary rock. It is thus concluded that the stratabound skarn of the Jiama deposit has nothing to do with the magmatic-hydrothermal solution, but is associated with the old seafloor hydrothermal circulatory system. It should genetically belong to hydrothermal exhalation. The study has provided valuable evidence for the origin of the Jiama copper and polymetallic deposit.