Abstract: A relevance theory among the interfacial deformation-secondary flow-second normal stress difference was established by computational simulation, and the mechanisms of interfacial deformation induced by the second normal stress difference driven secondary flow was proposed. The investigation indicated that the interfacial morphology evolved from concave downward to concave upward when the Giesekus model parameter alpha increased from 0.1 to 0.45. However, in the side wall region, the second normal stress difference of the melt along the interface became negative. If the absolute value of the second normal stress difference at the lower layer was greater than that of the upper melt, the secondary flows were all downward movement along the die wall, where a downward concave interface deformation were induced. Oppositely, the secondary flows are all upward movement along the die wall, where an upward concave interface deformation was induced. The absolute value of the second normal stress difference for the upper melt increased with an increase of alpha, whereas the melt of the lower layer increased at first and then tended to decrease. This lead to the downward movement of the secondary flow. The upward movement of the secondary flow was resulted by a value of alpha less than 0.25. However, when alpha was greater than 0.25, the positive and negative evolution of the interface deformation was induced definitely.