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.