Abstract: The primary cavity is a key region and one of the difficult items to the construction of Pebble-bed Modular High Temperature Gas-cooled Reactor (HTR-PM) demonstration plant. Modular design of primary cavity was studied in detail to enhance the construction quality and efficiency. The bracket modular design was adopted by the comparison of various design schemes. The modular division and design principles were proposed, the modular design of this region was completed, and a three-dimensional model for each module was built by using the three-dimensional design software CATIA. Analysis and calculation of strength and thermal stress of module support structure were completed. Construction feasibility and effect analysis show that the design scheme is feasible and the effect of reducing construction duration is obvious. The research has been applied in HTR-PM engineering design, effectively increasing construction efficiency and significant economic benefits have been obtained.