The feature of the flow front for polystyrene melts in injection molding is investigated in detail in this article. To explore the generation and evolution processes of "V"-shape pattern in the skin areas for polymer melt, a mathematical model based on the co-injection molding is proposed in which the evolution of material line is traced by level set method, and viscoelastic behavior of polystyrenes is described by Rolie-Poly constitutive equation; the molecular orientation, extension and deformation of polystyrenes are also studied in this article. The capabilities of Rolie-Poly constitutive equation is validated by simulating the principal stress difference(PSD) for both an entry-exit slit flow and a cross-slot geometry for polystyrenes melt; then the mold filling process has been analyzed by finite volume method to solve the continuity, momentum, constitutive equation together with level set equation. The numerical results for the evolution processes of "V"-shape pattern shows good agreement with available numerical results and experiments results obtained by injection machine. Furthermore, three main factors that include inlet velocity, molecular weight of polymer and temperature that affecting the formation and form of "V"-shape pattern were discussed, and molecular conformation of polystyrenes melt are also presented.