Abstract: Based on the classical lenticular model，with a positive-negative-pre-overlap method，combined with visualization technology，the cross-section deformation rule is studied in this paper. “Concave convex alternate model” is presented and its mathematical function is deduced. The research shows that the tow‘s concave cross-section alternates with the convex one periodically in length. The classical “invariant cross-section assumption” is amended，which has dominated the academic world for many years. A breakthrough has been made in the cross-section research.