Abstract: The driving force of primary loop of natural circulation reactor is limited and the total flow of loop is small, so the design and optimization of core flow distribution are very important. Reasonable core flow distribution can not only meet the thermal safety requirements, but also directly improve the core performance. Based on the above reasons, the optimization of flow distribution in natural circulation reactor was studied preliminarily. A onedimensional flow and heat transfer model was used to establish the initial solution model of inlet resistance coefficient optimization, and an exact solution searching algorithm was designed for the closed parallel channel studied in this paper. Coupled with COBRA, a core thermal analysis program was developed to optimize core flow distribution. By using the program, a natural circulation reactor was chosen as an example to calculate and analyze the flow distribution optimization in the life of the reactor core. The results show that, by taking the average value of the optimal setting scheme of the inlet resistance obtained by the optimization of the flow distribution of a single node in each typical life cycle, a better setting scheme of the inlet resistance in the core cycle life can be obtained. Aiming at the disadvantage that it is difficult to obtain the global optimal solution by means of the artificial design method of average value, a method of automatically realizing the optimization of flow distribution in the cycle life was proposed by referring to the modern optimization calculation method.