A comprehensive polymerization kinetics model that includes site formation, chain initiation, chain propagation, chain transfer and chain deactivation elementary reactions was incorporated into the three‐fluid computational fluid dynamics (CFD) model to describe the polymer properties in the gas–liquid–solid three‐phase polyethylene fluidized bed reactor. The effect of operating conditions on polyethylene properties was firstly discussed. Furthermore, due to the importance and complexity of scaling up three‐phase reactors, the incorporated CFD model was then employed to investigate the scale‐up influence, which indicates that the difference of flow hydrodynamics in the reactor due to scale‐up effect will result in the difference of polymerization reaction rate.