Hot washing on power transmission line devices is a convenient and effective way with high economic benefits for avoiding power grid pollution flashover.Technologies of the high-altitude hot washing devices worldwide are briefly described.Based on vehicle hot washing devices，a security protection system is designed，which contains multiple kinds of distance measuring technique，electric field distance prediction warning technique，a video monitoring system and so on.Distance measure and aiming function between the water gun and the insulators is achieved through the video monitoring system presented.Safety protection for the water gun platform is realized through ultrasonic distance measurement.After two-dimensional field intensity computation around the high voltage transmission lines with the charge simulation method，the probable accurate location between transmission lines and field measurement position is obtained through the field strength vector location method presented.The relative position and the nearest distance between washing devices and transmission lines are acquired by the proposal of the two field measurement way.Finally，the simulation of field strength vector location about 220 kV transmission lines is made with verification made by site actual test.