The local hot region possibly happening in the fluidized bed under unfavorable operations will lead to deterioration of fluidized states during fluidized beds start up and shut down，so it is important to fully understand the non-steady state of fluidized beds.The experimental study on the effect of fluidizing flux change on solids concentration was carried out in a two-dimensional fluidized bed of 15 mm thickness，400 mm width and 1200 mm height.The glass beads of average diameter of 0.45 mm were chosen as solids phase and water as liquid phase. The experimental results showed that there was an abrupt change of dense-dilute concentration interface with a continuous transition zone in the fluidized bed during the bed collapse process due to decreasing fluidization flux，while there was a continuous dense-dilute concentration interface during the bed expansion process with increasing fluidization flux.The concentration change during the expansion process of fluidized bed was larger than that during the bed collapse process of fluidized bed，and the response time of the expansion process was longer than that of the collapse process. A smaller change of fluid flux in the same time period led to a slower change of average concentration in the bed，which was unfavorable to forming the abrupt change of dense-dilute concentration interface during the collapse process of fluidized bed.A longer time period for the same flux change(slower change of fluidizing flux)led to a slower at first and faster afterwards change of average concentration while it is unfavorable to forming the abrupt change of dense-dilute concentration interface during the collapse process of fluidized bed.