A separating fluidized bed (SFB) withmagnetic powders as the heavy medium was used for dry coal separation. However, during the continuous separation process in themodularized dry coal separation system, the bed density under the coal inlets was found to fluctuate seriously. In this study, the effect of feed with different densities on the fluidization of the SFB was investigated using pressure time series and image analyses. The pressure measured at different bed heights severely fluctuated when particles run into the bed, and the bed surface sunk locally. After feeding, the SFB returned to the previous status, being a “separating fluidized bed”. However, for large particles, the stability of the SFB was severely damaged, and the “separating fluidized bed” acted as a “bubbling fluidized bed”, after feeding. Therefore, particles large enough to damage the stability of the SFB should not be fed into the bed.