Solids volumetric flux (V-s), which is expected to replace the solids mass flux (G(s)) as the key factor influencing solids holdup in circulating fluidized beds, was proposed to investigate the solids holdup variation and predict the extreme operation conditions in the downers. In this work, by using more than 400 sets of experimental data from different literature in Response Surface Methodology (RSM) analysis, it is found that the overestimation of particle density effect on the solids holdup can be avoided by converting the G(s) into V-s. Further studies indicated that the difference of solids holdup (Delta epsilon(s)) resulted from the different types of particles increased with the increase in V-s as well as the decrease in superficial gas velocity (U-g). Based on the normalized fitting results, the effects of different factors on the solids holdup were characterized. It is concluded that the effect extent was in the order of V-s > U-g >> particle size (d(p)) > particle density (rho(p)), which provided a guidance to control the solids holdup to satisfy the requirements in the downer by adjustment of various parameters, that is, V-s and U-g can adjust solids holdup roughly whereas d(p) and rho(p) can adjust solids holdup finely. Meanwhile, it is found that the upper limit of solids volumetric flux (V-s,V-max) can be increased by the increase of U-g, d(p) and/or rho(p). In addition, it is considered that using V-s to replace G(s) to define high-density operation in the downer could be more suitable in this study. (C) 2019 Elsevier Ltd. All rights reserved.