High-flux/-density circulating fluidized bed (CFB) technology has been widely applied to oil and coal processing with high solids flux (up to 500-2000 kg/(m(2) s)) and high solids holdup (up to 10%). However, the question of how to achieve high-density operations has not been reported systematically. This paper shows the effects of main parameters on high solids flux/density operation based on the systemic experimental work carried out under a wide range of operating conditions with superficial gas velocity of 5-9 m/s and solids circulation rate of 100-1300 kg/(m(2) s) in a specially designed riser 18 m high with fluid catalytic cracking particles. High solids flux and solids holdup, respectively, up to 1300 kg/(m(2) s) and 0.16 are achieved in the entire CFB riser showing that the high-density operating conditions are obtained. Additionally, the solids-to-air mass flow per meter can be used to judge the high-density operation along the entire riser in different CFB systems using the similar particles. The effects of the solids inventory, superficial gas velocity, aeration air in the storage tank, and the opening of the control valve on solids flow are investigated and discussed in detail. Based on the above work, an empirical model for prediction of the circulation rate of solids is established, which shows a good agreement with the experimental data in rather wide operating conditions. To find the key points to get high solids flux and high-density operating conditions, the loop pressure balance around all of the subunits of the whole system is well studied and a mathematical model for pressure balance distribution is developed, which can offer good guidance for the design and operation of the high-density circulating fluidized bed system.