Abstract: In this paper, applying target sandbox modeling results of “uncoordinated extension” and the new mechanical model of brittle faulting-“non-coordination criterion”, the controlling factors of fault formation and evolution are analyzed, and a new model of fault formation and evolution, which is called the “uncoordinated extension” model, is proposed. According to the model, the progressive deformation process of “uncoordinated extension” is the root cause which leads to the complexity of fault system in rift basins. Because the activity of each pre-existing fabric (mainly pre-existing fault) of different trend, property and size in the basement is different, it will lead to different, controlled faults in the strike, formation order and size and the complicated fault assemblage forms. “uncoordinated extension” is the basic patterns of fault formation and evolution, the complicated fault system can be formed progressively in progressive extension process under “uncoordinated extension” with unchanged extension direction. It is pointed out that the fault system in the rift basin is relatively complex, but the fault formation and evolution is orderly, and there is a law in the fault distribution, which can be analyzed and predicted with “non-coordination criterion”.