The invention discloses a tower cross arm reinforcing device. The tower cross arm reinforcing device comprises a rail slidably connected with a sliding plate, wherein the rail is fixedly arranged on tower diagonal rods through tower diagonal rod fixing structures arranged at two ends, a driving device with a locking structure is arranged in the sliding plate, and the sliding plate moves along the rail under the drive of the driving device; the surface of the sliding plate is hinged with one end of a supporting rod, and the other end of the supporting rod is fixedly arranged at the end portion of a tower cross arm through a tower cross arm fixing structure. By driving the sliding plate on the rail to move to the middle portion of the tower diagonal rod, then guaranteeing the position of the sliding plate to be fixed by using the locking structure and finally using the supporting rod to support the tower cross arm to form a new supporting structure, the load on the tower cross arm can be dispersed onto other rods through redistribution of corresponding force, the tower cross arm is effectively prevented from being damaged under severe conditions and thus the safety of the tower cross arm is guaranteed.