In order to investigate the instantaneous flow characteristics in the jet mixing zones of a circulating jet tank (CJT), a dynamic data acquisition system was employed to measure the instantaneous pressure fluctuation signals (PFS) in the Reynolds number (Re) range 3660-32940, at different axial, radial and circumferential positions. Frequency spectra were evaluated based on the local wave decomposition. The fluctuation energy distribution of instantaneous PFS was found to decrease markedly with increasing instantaneous frequency. For positions θ_m= π/6 and θ_m = π/3 the energy distributions were in the range 0-0.25Hz, while it was in the range 0-6Hz and 0-20Hz for θ_m= π/4 and θ_m =5π/12, respectively. The degree of fluid irregularity increased at low values of Re. Four concentrated energy bands at 32Hz, 25Hz, 7Hz and below 5Hz existed for Re=3660. However the energy was distributed in the range 0-8Hz and 0-3Hz for Re=18300 and Re=25620, respectively. With increasing values of z/H, the energy ratio in the range 0-1Hz initially decreased, then increased and finally decreased again. The energy was distributed in a relatively wide frequency brand affected by the free liquid for z/H=0.85 and the energy ratio at low frequency (0-5Hz) decreased.