Sound box capable of automatically adjusting size of sound cavity

The invention provides a sound box capable of automatically adjusting the size of a sound cavity, and the sound box comprises an outer cylinder, an inner cylinder is movably installed in the outer cylinder, the inner cylinder is fixedly provided with a loudspeaker, the inner cavity of the outer cylinder is provided with a motor seat and a first annular convex rib, the motor seat is provided with a fixing hole, a bearing is fixedly installed in the fixing hole, and the motor seat is fixedly provided with a driving motor. A long pipe is arranged in an inner cavity of the inner cylinder, a nut is fixedly installed at the bottom of the long pipe, a lead screw is movably installed in the long pipe, a second annular protruding rib is arranged on the outer side of the bottom of the inner cylinder, a second sealing ring is fixedly installed on the second annular protruding rib, the top of the inner cylinder is fixedly covered with a top cover, and a protection net is arranged on the top cover. When music is played, t