According to the theories of geometric optics and scalar diffraction, the refractive property of light for dual-wedge prism system was analyzed. The study found that the refractive effect of the dual-wedge prism rotating in the same rotation speed and in the opposite direction is equivalent to a single-wedge prism, of which an angle of refraction is changed with the rotary angle between two wedges. On this basis, a theoretical analysis on the diffraction light field of outgoing light through the rotary dual-wedge system was made and the law of position of diffraction light spots changes with the time under the situation of Fraunhofer diffraction was obtained. This research may give a useful theoretical direction for the 1-D controllable scan.