Abstract: The accumulator is mainly used to quickly inject borated water into the primary circuit when a large or medium loss of coolant accident (LOCA) occurs in a nuclear power plant, whose effective injection flow rate and continuous injection time have significant impacts on mitigating the consequences of LOCA. Based on the injection characteristics of accumulator during LOCA, physical model of HPR1000 accumulator was established in FLOWMASTER, and sensitivity analysis was performed on the resistance characteristics of the downstream direct injection pipeline, volume of the accumulator and initial pressure accumulation of the accumulator. Sensitivity analysis provided a basis for further optimizing the design of accumulators on the basis of meeting the requirements of safety analysis. Calculation and analysis show that reasonable selection of the pipe diameter and pipeline layout parameters of the direct injection pipeline and optimization of the initial pressure accumulation of the accumulator can improve the safety performance of the accumulator, further reduce the volume of the accumulator, and save the space of the reactor plant.