Abstract: Experimental tests were conducted to study the bed material balance in a cold circulating fluidized bed (CFB) test rig with breeches-legs, during which dynamic curves of air flow rate and bed pressure were obtained under different fluidization parameters. Based on theoretical model of parallel flow, the overturn condition in breeches-legs CFB was predicted, while the effects of control system simulated. Results show that the breeches-legs furnace will work in the self-balance zone if the fluidization air rate exceeds the critical value, in which case, the instantaneous deviation of air flow rate and bed pressure at both sides of the furnace will redress with one another and the bed material in both legs will be in dynamic balance. Bed inventory overturn occurs under the condition the total fluidization air rate being less than the critical data. The critical air flow rate decreases with rising static bed height. The method by adjusting valve opening to control the bed inventory overturn has its limitations. The most important way to control bed inventory overturn is to make the breeches-legs system operate in a self-balance state.