The hot-line cleaning of insulator contaminants has a great significance to prevent pollution flashover. The technique of hot-line cleaning insulator contaminants by water vapor is proposed in this paper. The flashover characteristics of insulator under the procedure of cleaning by hot-line water vapor are tested to investigate the security of cleaning system in the operation. Three kinds of insulator are chosed to carry on the experiment. The average flashover voltage

flashover voltage gradient and withstand voltage in35 kV insulators in hot-line cleaning are measured. The results show that the average flashover voltage and withstand voltage of insulators used for hot-line water vapor cleaning are both larger than the operation phase voltage in 35 kV system. The flashover voltage gradient is larger than usual pollution flashover voltage gradient. The system security can be ensured when there is no overvoltage in the system

which verifies that the insulators cleaning method presented in this paper is effective and reliable.