The transformation law of crystalline structure of polyacrylonitrile precursors in the process of fiber formation was studied by means of X-ray diffraction (XRD), and a new model for polyacrylonitrile precursor was presented. The results show that, in the process of fiber formation, the crystalline structure of polyacrylonitrile precursors are transformed from a two-phase semi-crystalline structure, with distinct semi-crystal (ordered) region and amorphous (disordered) region, into a single-phase para-crystalline structure, which is made by interlacing of the new ordered region transformed from amorphous region and initial semi-crystal region. In the single-phase para-crystalline structure, the polymer molecules are assumed to be a contorted helical shape to form rods (like zigzag), and the rods stack tightly via interactional overlapping, intersecting and intertwisting.