This study used multi-phase CFD (Computational Fluid Dynamics) technique to analyze the underwater radial impeller of the water circulator applied to the stagnant watershed. The impeller of the water circulation system has a function of generating atmospheric contact and flow by the jet on the water surface and generating a swirling flow so that the water below the surface layer can be transferred to the surface. Therefore, we applied the multi-phase technique to separate the atmospheric layer, the water layer, and the discharged jet flow layers in order to perform the CFD analysis that reflects the characteristics of the water circulator applied to this study. In addition, the impeller was designed to change the mode and vary the rotational speed according to the position at the water surface. In this study, CFD analysis for the optimization of the impeller was performed by changing the design values such as the angle and size of the wing, the inlet angle, and the outlet angle based on the model I. In addition, the VOF(Volume of Fraction) analysis of each model was used to analyze the discharge amount, angle, speed, and shafting force in steady state and unsteady state condition.