Before the thunderstorm occurs， the relative humidity of the air can reach 80%， and the surface of the insulator is wet and does not cause pollution loss. In this case， the insulator string is prone to lightning pollution flashover accidents， increasing the risk of equipment failure. Under the action of multiple factors， the internal insulation of porcelain insulators deteriorates to form zero-value insulators， resulting in further reduction of insulation performance. In this paper， porcelain insulators with a service life of more than 15 years in 35 kV transmission lines were selected to carry out lightning impulse discharge tests under wet pollution conditions at different zero values. The results show that the lightning pollution flashover voltage decreases with the increase of equivalent salt deposit density（ESDD）， and has a negative exponential relationship. The characteristic index of equivalent salt density of normal insulator string is 0.318， which is greater than that of zero insulator string. The lightning flashover voltage of normal insulator string is more significantly affected by the equivalent salt density. The position of the zero value insulator has a significant effect on the lightning flashover voltage， and the flashover voltage is the smallest when the zero value is located at the high voltage end under wet pollution conditions. After the installation of the large umbrella skirt， the influence of the equivalent salt density on the lightning flashover voltage of the insulator string is weakened， only 0.272. The location of the large umbrella skirt does not affect the dry arc distance of the arc， so it has a limited impact on the change of the pollution flashover voltage. After the installation of the large umbrella skirt， the pollution flashover voltage is improved， and the position of the large umbrella skirt has limited influence on the pollution flashover voltage. With the increase of the outer diameter， there is an obvious saturation trend. Under the same outer diameter， the lifting effect of the large umbrella skirt increases with the increase of ESDD.