The Dexing porphyry copper deposit, Jiangxi Province, is the largest porphyry copper deposit in eastern China. Previous studies indicated that the ore forming fluids were dominantly derived from the magmatic system, plus additional meteoric water, but there have been some controversies on the petrogenesis of the ore bearing porphyries and the tectonic setting and ore forming source of this deposit. Our new studies have suggested that the Dexing porphyries were emplaced in a continental arc setting coupled with the westward subduction of the Palaeo Pacific plate in the early Middle Jurassic（170.2～171.0 Ma）. Partial melting of the subducted slab (mainly overlying sediments), with subsequent melts interacting with the lithospheric mantle wedge, formed the high K calc alkaline porphyry magmas with high Sr, Sr/Y and La/Yb affinities. The large scale Cu—Mo—Au mineralization was nearly contemporaneous with the magmatism, and the ore forming source were extracted from the Fe3+ rich fluids/magmas released by the P MORB interacting with the lithospheric mantle wedge. However, during the middle Late Jurassic（～153.5 Ma）, Dexing area had been converted to a back arc or an intra arc extension environment triggered by the rollback of the palaeo Pacific plate, and there formed the ore barren dioritic intrusions with a crust—mantle mixing source. Moreover, there had formed a hydrothermal Cu (Au?) metallogenic event superimposed on the porphyry Cu—Mo—Au mineralization during the late Early Cretaceous.