Abstract: Gas-solid fluidized-bed dry separation technology offers the merit of being high in separation accuracy and has become one of the hot spots in research of dry separation technologies. How to keep the air bubble structure stably regulated and controlled remains one of the core problems. In order to explore the influence of selection of medium solids on motion behavior of air bubbles, different types of medium solids are selected for use, with an aim to analyze the characteristics of distribution of air bubbles in fluidized bed and the growth curve of motion of bubbles in medium solids fluidization process at the use of each type of medium solids, as well as the influence on fluctuation of bed density exerted by motion of bubbles. Based on results of analysis, an air-bubble growth prediction model is developed. Result of comparison shows the model has a high prediction accuracy. The study made in the paper provides a theoretical basis for improvement of particle fluidization process and elimination of the adverse effect on bed density produced by merging of air bubbles.