The invention discloses a tree-clearing device under power lines. The tree-clearing device comprises a telescopic rod. A fixed cutter head is mounted at the top of the telescopic rod, and connected with a movable cutter head through a rotating shaft. The movable cutter head is connected with a pull rope, the other end of the pull rope is connected with an operating handle on the lower portion of the telescopic rod, and the movable cutter head and the fixed cutter head are connected through a spring. When branches being cleared are hard and large in size, a mechanical device can be operated manually, so that transmission power is larger; when the branches being cleared are soft and small in size, the mechanical device can be operated electrically, so that working efficiency is high and convenience in using is realized.