Abstract: Pressure fluctuations are always the focus of discussion in fluidized field for a long time because their measurement is easy and ample information on fluidized state is included. Many investigators have studied fluidized-bed system by measuring pressure fluctuations. Recently, many researchers have recognized that fluidized-bed systems exhibit chaotic characteristics by analyzing pressure fluctuations. However, there are always noises in pressure fluctuations due to many unexpected reasons. As everyone knows, noises would affect the accuracy and result of chaotic computation and nonlinear analysis. So, different authors filtered firstly the noises by applying different threshold of low-pass filtering frequency such as Pence et al[11] , using 40 Hz as threshold of low-pass filter in order to eliminate high-frequency line noise, Bai et al[2] , Daw and Halow[12], using 20 Hz as threshold of low-pass filter, and so on. But no researches have been conducted to clarify what threshold of low-pass filter is exactly in pressure fluctuations. Because too low threshold of low-pass filter would make the information contained in pressure signals lose, and too high threshold of low-pass filter would induce noise and make a mess of analysis results.In this paper, the signals of pressure in abubbling fluidized bed, 0.30 m in diameter and 3.8 m in height are decomposed into signals of different frequency band including 0-10 Hz, 10-20 Hz, 20-30 Hz, 30-40 Hz, 40-50 Hz, 50-60 Hz and more than 60 Hz by Fourier transform. The linear and nonlinear correlation characteristics of signals of different frequency band are studied using auto- correlation function and mutual information function, respectively. It is the fist time that the signals of more than 30 Hz in frequency are confirmed to be not of linear correlation and the signals of more than 40 Hz in frequency to be not of nonlineal correlation . Therefore, the signals of more than 40 Hz in frequency could be considered as noise for no correlation in any time interval. Meanwhile, the signals of less than 40 Hz in frequency should be considered rather than 20 Hz as before if more details on fluidized bed are to be obtained by studying pressure signals.