The utility model discloses a section-varied tailfin of a racing car, which comprises an upper-layer primary flap and a lower-layer flap, wherein the upper-layer primary flap and the lower-layer flap are in parallel arrangement, lateral plates are symmetrically arranged at two sides of each of the upper-layer primary flap and the lower-layer flap, the upper-layer primary flap is connected with an upper-layer accessory flap, two ends of each of the upper-layer primary flap, the upper-layer accessory flap and the lower-layer flap are connected to the lateral plates, the middle portion of an upper surface of the upper-layer primary flap is an upward-raised cambered surface, radius of the cambered surface is gradually reduced in the width direction of the upper-layer primary flap, and the maximum radius of the upward-raised cambered surface of the upper surface of the upper-layer primary flap is 180-220mm. According to the section-varied tailfin of the racing car disclosed by the utility model, as the upward-raised cambered surface of the upper-layer primary flap is designed based on the position of a primary ring of a car frame, airflow in the middle part of a car body is utilized by the upper-layer primary flap to the maximum extent through the flow guiding effect of a variable section; compared with a racing car provided with a straight-plate type tailfin, the racing car with the section-varied tailfin has the advantage that airflow utilization rate of the section-varied tailfin disclosed by the utility model can be increased by 10%, and a down force generated thereby accounts for 33% of the down force of the racing car.