Although particle loading is often assumed to have a significant impact on fluid flow in cyclone separators, the specific effect can be confusing due to a lack of fundamental knowledge of the operating principles. The problem was addressed in this work by numerically analyzing the particle mass loading impact of different sizes on the flow within the square cyclone separator using the computational fluid dynamics (CFD) approach. This type of cyclone is an effective cleaning mechanism for high-temperature gases in a circulating fluidized bed (CFB) boiler. Therefore, it is also critical to investigate the effect of particle mass loading on gas flow at low and high temperatures, which has yet to be taken into account in the literature. Consequently, the current study focuses on this issue as a first step toward developing square cyclones by better understanding the influence of particle concentration on airflow. To describe particle flow, the Eulerian-Lagrangian technique was used to solve the unsteady Reynolds-averaged Navier-Stokes (URANS) equations. The discrete random walk (DRW) was employed to evaluate velocity fluctuations. The results demonstrated that as particle mass loading increased, the sweeping impact of enhanced larger particles caused the smaller particles to flow toward the wall region, increasing particle concentration at the wall region. The particle concentration at the bottom of the square cyclone increased 11 times when the particle mass loading increased from 6.9 to 41.7 g/m3. As the tangential velocity of the gas increased with particle mass loading, more particles accumulated at the bottom of the conical section and remained there for an extended length of time, increasing the chances of their separation.