The invention discloses a polishing device used after deep hole machining. The polishing device comprises a rotating device provided with a polishing base, a connecting rod coaxial with the polishing base is connected to the polishing base and is a stretchable rod, and a cylindrical polishing head coaxial with the connecting rod is connected to the lower end of the connecting rod; the diameter of the polishing head is larger than the diameter of the connecting rod; and a plurality of spherical polishing balls are embedded on the outer side wall of the polishing head, and the polishing balls are in rolling connection with the outer side wall of the polishing head. By means of the polishing machining device, materials used by the polishing head can be reduced, cost is saved, meanwhile, a connecting bar with a stretchable function is used, a deep hole can be sequentially polished from top to bottom, and therefore when the voice-activated upper half part is polished, the length of the connecting rod can be small, the draw ratio of the whole is small, damage to the polishing head is reduced, and the condition of polishing bar breaking easily caused during polishing when the overall deep hole is polished in the prior art is avoided.