Abstract: The existing supercritical water-cooled Reactor (SCWR) undertakes a design of water rod assembly to solve the problem of neutron moderator insufficiency. However, this brings a giant challenge to engineering feasibility of the assembly and reactor core designs. This article presents a design of utilizing an assembly with mixed moderators that adopts the coolant (not floated and heated through the fuel region) and the solid moderator in fuel region to achieve the design target of abolishing water rod completely and simplifying the assembly design of SCWR. On the basis of analysis and calculation, compared with the water rod assembly, the assembly with mixed moderators has advantages of simple design, high capability of economy and safety.