Abstract: Gas-solid circulating fluidized beds (CFBs) are widely used as reactors in industry with performances heavily relying on the interaction between gas and particles. In a CFB, some particles stay dispersed, while other particles aggregate to form dense and large clusters. As the gas-particle interaction is simple for dispersed particles, the prediction of reactor performances calls for a thorough analysis of the gas-cluster interaction. In the literature, cluster properties have been characterized extensively. However, how the gas interacts with clusters has not been quantified systematically yet. In this work, the gas-cluster interaction was quantified numerically by characterizing the penetrating gas flow into clusters. With 99 simulations, influences of the particle randomness, cluster solids holdup, cluster diameter, particle diameter, superficial gas velocity and fluid properties on the gas-cluster interaction were investigated systematically. Then, an empirical correlation was proposed to quantify the gas-cluster interaction, significantly contributing to the design and optimization of CFBs.