Abstract: The structure of water-rod is designed separately in supercritical water-cooled reactor. The water in water-rod makes the core well moderated. Based on the SCWR neutronic/thermal-hydraulic coupling code programmed by authors, the core design of Japan was studied. Considering three types of water-rods designed by ourselves, which named S type, D1 type and D2 type, and different side lengths of inner water-rod, the distributions of moderator density, average density and axial power under each condition were obtained. The results show that the D2 type has more uniform distributions of moderator density, average density and axial power. Axial average moderator density increases with the length of outer side of inner water-rod.