The invention relates to maintenance devices and methods of power transmission lines, in particular to a stress device for replacing the cross arm main material of a power transmission line tension tower and an application method. The stress device comprises a between-wire load bearing part, a temporary support cross arm arranged on the tower and a tightening tool, wherein load-bearing extension rods used for being connected with insulator chains are detachably connected to two ends of the between-wire load bearing part, and a temporary-support-cross-arm reinforcing part is connected between the temporary support cross arm and the tower. The stress device is simple to operate, high in operation safety and capable of saving time and labor.