Abstract: In order to solve the problem of excessively large boron concentration difference between the primary circuit and the pressurizer in the normal dilution and boration process of Improved Chinese Pressurized Water Reactor (ACPR1000), and to prevent the accidental violation of the operating technical specifications of the unit, the cause of the difference in the boron concentration was analyzed theoretically. RELAP5-3D program was used to simulate the reactor neutronics and the thermal hydraulics of the reactor coolant system, and the simulation platform was used to simulate the nuclear island auxiliary and related control systems. The full scope simulator composed of these system models was used to comprehensively analyze the influence of various factors, and quantitatively calculate the difference in the boron concentration under the rapid boration transient. Combined with the operating regulations and experimental data of the unit, the results show that the magnitude of the difference in the boron concentration is greatly affected by the spray and charging flowrate during the transient, and the current operating procedures may lead to that the units exceeds requirements of operation specifications. So that this paper proposes improvements.