Based on the brief introduction of characteristics of Mo grade, reserve, temporal and spatial distribution and tectonic setting on porphyry type Mo deposits in this study, we focus on two aspects, the first is the geochemical behavior of molybdenum in the process of magmatic—hydrothermal evolution, the second is the precipitation mechanism of molybdenum. We propose that the oxygen fugacity of oreforming magma from porphyry Mo deposits varies in a relatively wide range, but it is commonly higher than FMQ (fayalite—magnetite—quartz) buffer system. Furthermore, the formation of porphyry Mo deposits depend factors such as an enormous magma chamber, the concentration of giantvolume fluid in a small and shallow magmatic rock and the relatively high content of volatile in magma et al. Whereas it has no relationship with the Mo contents of oreforming magma and initial fluids. In the oreforming fluids, the decrease in oxygen fugacity and/or pH plays a vital role in the precipitation of molybdenite. Moreover, the temperature and salinity reduction also contributes to molybdenum deposition. In addition, multistage fluid boiling can be very important for enhancing molybdenum grade. Apart from hydrothermal stage, the studies in the process of magma crystallization differentiation and magmatic—hydrothermal transition should be strengthened.