There are many faults with different trends in complicated fault blocks in an extensional basin, and the fault assemblages are very complex. Such complicated fault assemblages are usually thought to be the result of superposition tectonic events of multiple stages in the past. However, according to the objective sandbox modeling of the complicated Zhangjuhe faulted block, the author found that complicated fault assemblages can be formed by means of progressive deformation due to unchanged tectonic forces. Based on the experimental results in combination with theoretical analysis, a new genetic model of complicated fault assemblages??the disharmonic extension-progressive deformation model is proposed, which describes complicated structures with simple origins. This model shows that preexisting structures, such as faults in basement, and basement movement have a key effect on the deformation of basin covers. When the regional extension is disharmonic with the basement structure and movement, complicated fault assemblages can be form during progressive deformation. This model provides a new idea for revealing the internal relationships of complicated fault assemblages in an extensional tectonic system.