Heavy rainfall can bridge the sheds of insulators on transmission lines, substations, and converter stations with continuous water columns, leading to electric field distortion. At the same time, the porcelain insulator is easily deteriorated due to its own weight and wind load during long-term operation. The formation of zero-value insulators leads to the occurrence of rain flashover accidents. To investigate the lightning impulse discharge characteristics of porcelain insulator strings containing zero value insulators under heavy rainfall, lightning impulse tests are conducted on XWP2-160 porcelain insulator strings under various zero-value configurations. The results show that, both with and without zero-value insulators, the lightning rain flashover voltage of the porcelain insulator string decreases with the increase of rainfall intensity and rainwater conductivity. The flashover voltage of insulator strings without zero-value insulators is more significantly influenced by rainfall intensity and rainwater conductivity. As the number of zero-value insulators increases, the influence of rainfall intensity and rainwater conductivity on the lightning impulse flashover voltage becomes smaller. When zero-value insulators are located at different positions along the string, the lightning impulse flashover voltage gradually decreases with increasing rainfall intensity and rainwater conductivity, showing a tendency toward saturation. When the zero-value insulator is arranged at the high-voltage end, the rain flashover voltage is obviously greater than that at the middle and grounded end, and the degree of influence by the rainfall intensity and rainwater conductivity is also greater than that at the middle and grounded end. The research results can provide important guidance for the design and configuration of external insulation systems of transmission lines to cope with thunderstorms.