Abstract: A method of slowing down the rotation of an asteroid with a tethered solar sail is proposed in this paper. With no change of the length of the tether, the solar radiation pressure suffered by the solar sail will keep it on the synchronous orbit of the asteroid. It can prevent the tether from twining around the asteroid caused by the rotation of the asteroid. By controlling the solar sail, the tether can be tightened. The force in the tether will continually provide a torque opposite to the rotation direction of the asteroid. Then the rotation rate of the asteroid can decrease. The results show that when the area of the solar sail is 10⁶ m², the rotation of the asteroid can be eliminated after about 86 days. It verifies the effectiveness of this method.