The invention discloses a rotary cup which comprises a cup body and a cup shaft, wherein the cup body is composed of a cup bottom and a cup edge; the cup edge and the cup bottom are crossed and formed into a condensation slot; the bottom surface of the cup bottom is equally divided into two halves; at least one set of narrow seams is arranged on one half part; 2-3 narrow seams are contained in each set of narrow seams; a damping block is embedded into the condensation slot along the peripheral direction of the cup bottom on the other half part; the narrow seams pass through the cup bottom. A yarn which is spun by utilizing the rotary cup provided by the invention is in a special style of controllable change in longitudinal yarn density, a tight and high-twist thin part and fluffy and a low-twist rough part. The appearing law of the rough thin sections is consistent with the embedding law of the seams and the damping block of the rotary cup.