V-type insulator string has been widely applied in 500 kV straight line tower. However, damaged and dropping accident of insulator string occurred frequently, because of the compression on V-type insulator string at leeward side. The relationship between Kv value of transmission tower and V-type insulator string loadcarrying capability was obtained by means of computation. The actual Kv value could be adjusted more than the control value in the design stage of transmission lines, in order to avoid the damaged and dropping accident of insulator string, which could provide guiding significance to the pole position for transmission lines.