Abstract: A CFD method is used to simulate the flow field of double loop In-containment Refueling Water Storage Tank (IRWST) in a nuclear power plant, and an uniformity criterion number and volume ratio of low velocity region is adopted to quantitatively evaluate the mixing performance of IRWST. Results indicate that the comprehensive mixing performance of double loop IRWST is better than European Pressurized Water Reactor (EPR) nuclear power plant by reasonable arrangement of mixing pipes, which ensures the availability in engineering application. Further two structural optimizations are proposed, and the results show that the mixing performance of IRWST can be improved by reducing the diameter and adjusting the direction of mixing pipes.