A high-density circulating fluidized bed (HDCFB) has broad prospects in catalytic processes. It can handle higher solid circulation rates than a low-density CFB (LDCFB), leading to more complicated clustering phenomena that are challenging to describe theoretically. This paper presents a two-fluid model facilitated with the kinetic theory to simulate HDCFBs. To improve the predictability of such a model, an existing solid pressure correlation is empirically modified based on experimental data to consider the influences of different clustering behaviors inside the LDCFB and HDCFB. The validity of the model is first verified by comparing the numerical results with the measurements in terms of gas pressure and solid holdup and velocity under various conditions. Then, the effects of superficial gas velocity and solid circulating rate on cluster formation and associated gas-solid flow are studied. The results demonstrate the differences between the LDCFB and HDCFB in cluster size and shape and flow heterogeneity.