Abstract: Improved assembly and core design were proposed and analyzed using 3D neutronic/thermal-hydraulics coupling calculation system. In the assembly design, the optimization for the fuel rods with burnable poison was carried out to minimize the remains of burnable poison at the end of the cycle. In the core design, the water flow scheme, fuel loading pattern and control rod loading pattern were optimized for extending the core life as well as decreasing the maximum cladding surface temperature. The results of coupling calculations show that the equilibrium core satisfies all design criteria, and the main core parameters are superior to that of previous study.