The important role of tail fin of a fish in swimming is to produce a thrust. In this paper,a three-dimensional test and analysis method for instantaneous 3D tail fin profile measurements based on the Fourier transform profilometry was proposed to understand the function of the tail fin. By taking a carp in cruising for an example,the moiré fringes were projected onto a carp tail fin to produce the deformed fringe patterns containing 3D information. Then,a high-speed camera was used to capture these time-sequence images. After Fourier transform,filtering,inverse Fourier transform and unwrap for these phase maps in a 3D phase space,a instantaneous 3D profile model was established and the complex tail fin movements were really reconstructed. Through the analysis of displacement-time curve,the tail-beat frequency is calculated to be 1.42 Hz. It is concluded that the presented study provides a new method both in study of the kinetic characteristic and in quantified analysis of change processing for the tail fin during steady swimming.