This study was conducted to evaluate the ratio of the pressurized flow to the main inlet flow, which has been considered one of the most important parameters for operating the pump diffusion mixer (PDM). Computational fluid dynamics (CFD) simulation was employed to evaluate the conventional operation rule of PDM and to propose a supplementary operating parameter and criterion. Test simulation of CFD was carried out for the 21 cases of flow ratio in a full scale PDM. The values of local velocity gradient were calculated in each case to analyze the simulation results in more detail. A wet test was conducted to verify CFD simulation results, which measures the factual coagulant dispersion distribution at a distance of 5.4 m from deflector. From results of both CFD simulation and wet test, the flow ratio was adequate as an operating parameter or criterion; also, the velocity ratio (dimensionless) of the pressurized flow to the main inlet was useful in predicting the performance of PDM. In addition, the injected coagulant could be dispersed evenly in overall cross section on the condition that the velocity ratio is at least over 20.