The fluid system H2O-Cl-S is the most typical representative of porphyry-epithermal-Cu-Au ore forming fluids. The properties of fluids of interest dominate the behavior of Cu, Au in the fluids, including solubility, spicies, and liquid-fluid partition. The species formed responsible for dissolving and transporting metals are determined by oxygen fugacity and Cl/S ratio of the fluids. High temperature, salinity and oxygen fugacity favor Cu-Cl and Au-Cl complexes in the fluids, where S-3 is likely to serving as an important S bearing species for dissolving Au. Excessive amount of S with respect to Fe and Cu is benifical to distributing Cu and Au by the form of charged ionic and neutral complexes in liquid like and vapor like fluids, respectively, the latter of which would migirate Au as far as epithermal enviroment. The source magama ought to experience adequate fractionation to exsolute fluids with components and properties that are so good for transportation of metals that high grade porphyry Cu, Au could form. Especially in the case of space-time related epithermal Au over the porphyry system, it is required that initial state of the fluids is in the supercritical region of the referential binary system of NaCl-H2O; efficient fluid evolution patterns, favorable buffering surroundings and suitable sites for precipitation of Au are encouraged as well. Phase separation and fluid-fluid reaction are the two most important manners determining the precipitation of porphyry-epithermal Cu-Au. Vapor like fluids have characteristic properties and evolving pattern, which should be considered as an important ore forming fluid.