Sample entropy (SampEn) and intrinsic mode entropy (IMEn) are used to evaluate the fluctuation characteristics in a novel circulating jet tank (CJT). SampEn analysis indicates that the fluctuation complexity of intrinsic mode functions (IMF) reaches a maximum in the first scale and then decreases with the increase of the decomposed scales. When the frequency bands are less than 36.25Hz, the fluctuation complexity of the IMF gradually increases with increasing Reynolds numbers. The IMEn of IMF with different cumulative scales decreases and then increases with increasing Re. The IMEn of pressure fluctuations is dominated by the functions IMF1 to IMF4. IMF5 to IMF7 start to play a more important role in the fluctuation characteristics for Re>29280.