Abstract: A large-scale and refined three-dimensional numerical simulation of the upper plenum flow field of the pressure vessel of the three-loop pressurized water reactor is carried out by using the commercial CFD software STAR-CCM+ code. The coolant flow at the outlet of 157 fuel assemblies is calculated by the component tracking method. A mixing matrix of the upper plenum with 3×157 elements is constructed, which can be used to quantitatively and accurately describe  the complex flow process of the coolant flowing out from the reactor core and mixing in the upper plenum and redistributing to the hot legs. It is found that the mixing of the coolant flowing from the core is not thorough and complete in the upper plenum of pressure vessel. The coolant flowing from fuel assembly at different positions in the radial direction is with obvious corresponding relationship in the interface area between the upper plenum and the hot legs, and the difference of the radial power distribution of fuel assembly will inevitably lead to the formation of the thermal stratification of the coolant in hot legs.