The normal stress between wires is one of the main factors of the slipping and stress concentration in the stay-cables. In order to improve the research on the stay-cables, the mechanism of the normal stress has been studied in this paper. The normal stress can be resulted from two reasons. Firstly, the normal stress generate from the semi-parallel wires because of the existence of spiral angle. Secondly, the normal stress results from the pinch-effect generated by the hot-extruding PE protection. Both the formulation have been listed to analyse the normal stress in this paper. Result shows, the normal stress between the outermost layers is far smaller than the inner layers. Finally, the field survey surveys have been used to verify the analysis.