A pressure balance analysis has been carried out, along with a dimensionless empirical correlation for the solids holdup in the riser and a semi-empirical equation for the pressure drop across the non-mechanical control valve derived in this work, to successfully predict the stable operating conditions and explain the origin of the unstable operation phenomena of the liquid-solid circulating fluidized bed. The effects of the total and auxiliary liquid velocity, the solids inventory and the unit geometry on the stable operation range are discussed. Furthermore, this work also reveals the importance of unit structure in improving the performance of the liquid-solid circulating fluidized bed systems.