Pressure fluctuation measurements were conducted in a pressurized spout-fluid bed of 100 mm in diameter with bed pressure up to 0.4 MPa. Information theory based Shannon entropy analysis of pressure fluctuation was developed to recognize chaotic nature of gas-solid flows. It was found that both Shannon entropy and Shannon entropy increment rate were pronounced with the flow patterns and transitions. Moreover, Shannon entropy increment rate could describe the change of chaotic nature with operating conditions. Increasing chaotic nature was observed with increasing fluidizing gas flow rate. When elevating bed pressure, stable flow regions became large and the chaotic nature was weakened.