[en] By analyzing the behavior and existence form of uranium in different geochemical environments, existence form of uranium and uranium minerals species, this paper expounds the formation mechanism of main commercial uranium mineral--pitchblende: (1) uranium is a valence-changeable element. It is reactivated and migrates in oxidized environment, and is reduced and precipitated in reducing environment; (2) [UO₂(CO₃)₃]^4-, [UO₂(CO₃)₂]^2- coming from oxidized environment react with reductants such as organic matter, sulfide and low-valence iron at the redox front to form simple uranium oxide--pitchblende; (3)the adsorption of uranium by organic matter and clay minerals accelerates the reduction and the concentration of uranium. Therefore, it is considered, that the reduction of SO₄^2- by organic matter to form H₂S, and the reduction of UO₂²⁺ by H₂S are the main reasons for the formation of pitchblende. This reaction is extensively and universally available in neutral and weakly alkaline carbonate solution. The existense of reductants such as H₂S is the basic factor leading to the decrease of Eh in environments and the oversaturation of UO₂²⁺ at the redox front in groundwater, thus accelerating the adsorption and the precipitation of uranium