The spent fuel reprocessing plant is the core link to realize the closed cycle of nuclear energy and plays a pivotal role in the whole nuclear industry system. However, the current safety management system of spent fuel or high radioactive waste liquid is not perfect, there are potential threats in the spent fuel reprocessing plant in the aspects of facilities, transportation and safe storage of radioactive sources. In order to take the most appropriate preventive measures to improve the overall safety of the reprocessing plant, it is urgent to carry out the analysis of the nuclear security incidents and the importance of the construction of the spent fuel reprocessing plant. In this paper, the importance of nuclear security incidents is analyzed by fault tree method. Eight types of nuclear security incidents with high probability in spent fuel reprocessing plant are determined through investigation and the probability of basic events is obtained by using expert scoring and probability-mathematical statistics processing method. In order to improve the reliability of statistical data, the evaluation results of each expert are given corresponding weights by the assignment method because of their different qualifications and work experiences. Secondly, the importance of the buildings is analyzed by the method of fuzzy decision method. This paper mainly studies the main process area and the three-waste area of the reprocessing plant, which is further subdivided into seven units, the expert scoring results are used as the judgment matrix. The results of comprehensive decision-making are processed by mathematical statistics. Therefore, seven units can be obtained. The importance of the main process area and the three-waste area are calculated by the method of summation and average. The results show that the highest importance of computer nuclear security incidents is 0.193 and the lowest importance of theft incidents is 0.018. Therefore, more protection efforts should be put into the computerized nuclear security incidents. The maximum importance of the transformation plant unit at the tail end is 0.561, which has a significant impact on the overall safety of the spent fuel reprocessing plant. The importance of the main process area (0.153) is higher than that of the three-waste area (0.135), indicating that more safety protection forces need to be invested in the main process area. This study can provide theoretical support for the design of nuclear security action plan and the safe operation of spent fuel reprocessing plants.