According to the functional requirements of the passive core water replenishment system, in the event of a nuclear power plant accident, it is required to inject high concentration boron-containing water into the reactor core for cooling and dilution through the driving force generated by the density difference between the inlet and outlet pipelines of the CMT. This function requires the connection pipes of the CMT to achieve cold trap effect, effectively isolating turbulent penetration and temperature diffusion from the high-temperature coolant in the primary loop; At the same time, it is required that the boron concentration in the makeup tank during the refueling cycle reaches the set limit or above, in order to avoid adverse effects on the reactivity of the core after coolant injection. This article uses CFX numerical simulation software to simulate the cold trap phenomenon and boron acid solution mass diffusion in the passive core water replenishment system. The rationality of the pipeline layout scheme and the distribution of boron acid solution are determined to meet the relevant limit requirements for core safety during the refueling cycle. Based on the characteristics and mechanisms of the system pipeline, optimization design directions and reasonable suggestions are proposed.