Abstract: In order to develop a kind of standard calibration borehole,studies on acoustic wavefield in a fluid-filled borehole with outside traction-free cylindrical boundary conditions are intensified.Dispersive characteristics of various symmetric mode waves in this kind of cvlindrically closed waveguide structure are calculated and analyzed.Numerical results show that,the dispersion curves of the fluid-filled borehole go across between the dispersion curves of the empty borehole and the fluid cylinder.The so called head velocities calculated by the Slowness-Time-Coherence(STC) method have a great agreement with the values in the "plateau" range on the phase velocity dispersions curves.When the window length is 4 to 5 times of the wave cycles,the difference between the "plateau" velocity and the longitudinal velocity of the borehole material increases exponentially with the incensement of the Poisson's ratio of the borehole material.It is also shown that the size of the borehole radius could be chosen for obtaining the appropriate plateau width,location and smoothness.This work may provide a new sight for designing borehole acoustic logging calibrations.