It is widely accepted that porphyry type deposits form from some oxidized ore fluids. 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.