Abstract: Flapping lunate tail is studied through hydrodynamic test in water channel,and proper vorticity control methods for flapping foil propulsion are discussed based on the principal vorticity parameters,such as Strouhal number(St),angle of attack,the dimensionless transverse amplitude,and phase difference.Free-swimming test is carried out on an unmanned underwater vehicle with flapping tail.The results show that power consumption of vehicle can be reduced effectively through active control of St and other vorticity parameters using fuzzy control methods,and greater swimming speed can be obtained.The maximum swimming speed as high as 1.87 m/s is measured.Meanwhile,St of flapping tail is controlled within the range of 0.4～0.5.