It is widely accepted that porphyry type deposits form from some oxidized ore fluids. But recently, some typical ilmenite-bearing, reduced I-type granite related porphyry Cu-Mo-Au deposits with relatively reduced ore-forming fluids are found, in which primary hematite, magnetite and sulfate minerals are absent. The ore-forming fluids of these deposits always contain abundant CH4 associated with CO2. According to related research results, the CH4 is proposed to be derived from the contamination of S-type granite into I-type one, but it is also possible that CH4 comes from the degassing process of the Earth. A chemical model is set in order to interpreting the role of reduced fluids in porphyry metallogenesis. According to the model, H2S and CO2 are derived from the reaction between CH4 and SO2 both of which come from one evolved igneous system, which will cause the precipitation of the molybdenite and cogenetic calcite. While the Cu and Au will be transported by the reacted fluids under reducing conditions and deposit in some cites far away from the granite system, the process will form a cogenetic subclass porphyry Cu-Au deposits.