Abstract: The mixer-settler is a stepwise contact liquid-liquid extraction equipment widely used in the spent fuel reprocessing and rare earth industry. The materials and solvents are mixed with each other in the mixing chamber through stirring for mass transfer, and then enter the clarification chamber for separation by gravity. The fluid mechanics performance of the mixer-settler has an important impact on the hydraulics stability, treatment capacity and mass transfer efficiency of the equipment operation. Many scholars conducted research on mixer-settler based on fluid experiments, and focused on simulation calculations of single parts such as mixing chambers, clarification chambers, or stirring paddles. There were few reports on CFD simulation of the entire production scale full countercurrent mixer-settler, which inevitably lacked the research about the influence of the combination of mixing chambers and clarification chambers. Moreover, existing research mostly focused on single-phase systems or liquid-liquid two-phase systems, there is limited research on the simulation of gas-liquid (aqueous phase)-liquid (organic phase) three-phase systems. In this paper, the behavior of gas-liquid (water phase)-liquid (oil phase) three-phase fluid at the micro level in the industrial full countercurrent mixer-settler was studied by CFD method. The mixer-settler is divided into two grid structures, the validation on grid independence analysis was carried out, and the simulation results were analyzed and discussed. The results show that mixing will produce a V-shaped liquid level distribution in the mixing chamber, and there will be a negative pressure area behind the blade and cavitation in the upper part. The pressure stagnation dead zone will appear at the four corners of the square mixing chamber. The oil phase in the mixing chamber will have a coating layer around the mixing shaft, and the water phase will be mixed with the oil phase in the blade area after being inhaled directly below the blade. There are multiple local velocity vortex fields in the mixing chamber. The lower part of the blade is the flow of the surrounding liquid directly below the blade, and the upper part of the blade is the flow of the upward and backward rotating shaft. The mixing of two phases and the breakup and coalescence of liquid droplets mainly occur at the blade and on the wall near the mixing chamber. Besides, when using Eulerian multiphase flow simulation, there may be a certain degree of discrete error due to software calculations. Reducing the volume fraction relaxation factor can reduce the error, but it can’t be avoided. When considering computational efficiency, the gas phase can be set as the main phase to reduce the error. At the end of this paper, some optimization methods and suggestions were given to improve the operation of the mixer-settler.