Abstract: In the design of HPR1000 reactor structure, in order to improve the structure safety, the in-core instrumentation detector is guided out of the reactor from the vessel head instead of the lower plenum. The structure of the lower plenum has been changed, which affects the uniformity of the inlet flow field, so a new lower plenum structure is needed. The reactor lower plenum structures of CNP1000, AP1000 and EPR are compared. Based on the characteristics of HPR1000, four schemes for optimum structures are presented. By modeling and CFD analyzing, the four schemes and that for CNP1000 are contrasted and analyzed in terms of structure, manufacture, assembling and flow field analyzing. It is found that the program which uses flow distribution plate is the best, and the uniformity of coolant mixing and flow distribution at the core inlet can be satisfied.