The temperature and flow rate of coolant in the hot legs of pressurized water reactor (PWR) systems directly reflect the nuclear power and the heat transfer state of the reactor core, and are key parameters for reactor power control and safety protection. In order to comprehensively understand the distribution and evolution of the coolant flow-thermal coupling field in the upper plenum and hot leg of the Hualong One, and provide references for the measurement and control of core parameters, FEA (finite element analysis) method was employed in this paper to conduct CFD (computational fluid dynamics) numerical simulations of the coolant flow region in the upper plenum and hot legs. Firstly, a reasonably simplified 3D geometrical model of the upper plenum and hot legs of the Hualong One was established. Subsequently, the computational domain of the model was discretized into meshes and a mesh sensitivity analysis was performed. Finally, through calculations, a steady-state solution of non-isothermal coolant flow was obtained, with relative errors between flow rate, temperature and related design estimates and actual measured values all less than 2%. Analysis of the steady-state characteristics indicates that an uneven coolant temperature distribution at the inlet of the hot legs is caused by insufficient heat exchange between high and low temperature coolants near the vertical inner wall of the upper plenum, with a temperature difference between 14.0 ℃ and 16.3 ℃. As the coolant flows along the axial direction, both the temperature and flow distribution gradually become uniform and stable. Furthermore, the variation of the coolant temperature distribution is dominated by the flow of the low temperature coolant inside the hot legs.