This paper introduced the specified value of composite insulator’s design safety factor in the design code for overhead transmission line, and compared the force condition of suspension string and tension string in different conditions, such as ice, strong wind and annual average temperature. According to the comparison of meteorological data in several typical areas, this paper pointed out that the design must be different between tension string and suspension string, due to the operational condition of tension string was worse than that of suspension string, and the creep in operation life period of tension string was more serious. Finally, the value of design safety factor was suggested for composite insulator tension string.