The present invention discloses a composite cross-arm insulator device and method for replacing an overhead transmission line. The device comprises a wire support frame which has one end provided with a pulley block lifting point for installing lifting wires and the other end provided with a U-shaped hoop configured to fix the support frame on the steel tube tower overhead grounding cross arm; and the wire support frame is provided with a reinforced tripod configured for improving the intensity of the wire support frame. The device structure is simple, the installation operation is convenient, the wires are effectively lifted and fixed, the wire shedding is avoided, and the efficiency is fully improved. The composite cross-arm insulator device and method for replacing an overhead transmission line may be used for replacing a composite cross-arm insulator for one time in the multi-loop overhead transmission line erected by the steel tube tower so as to effectively reduce the number of times of power failure, ensure the power supply reliability, realize the economic benefit and create more economic values for the society.