Abstract: Flexibility plays an important role in the movement of organisms, such as the flight of birds and the movement of fish. The study of flexibility is conducive to a profound understanding of some animal motion mechanisms and the production of highly efficient bionic machines. Based on the model of lateral free elastic filament, the aerodynamic efficiency of different flexible filaments was studied in this paper. The numerical results show that the drag coefficient of the filament decreases with the decrease of the flexibility, and the amplitude of the tail end of the filament increases with the decrease of the flexibility in Kárman vortex street. The results also show that in a certain range of Reynolds number, the filament swings from the swinging motion mode between the vortex cores to the swinging motion mode outside the vortex street with the decrease of the flexibility. The symmetry of the wake also changes significantly.