The　fuel-assembly　bowing　is　widely　observed　in　PWR,　due　to　the　complex　operational　environment　in　the　core.　After　the　fuel-assembly　bowing,　the　three-dimensional　(3D)　water-gap　distributions　around　the　fuel　assemblies　would　be　deviated　from　the　nominal　values.　From　the　view　of　neutron　physics,　the　variations　of　water　gap　would　result　in　over-moderation　or　under-moderation　of　the　neutrons　locally,　hence　impact　the　results　of　the　neutronics　simulations.　In　order　to　estimate　the　neutronics　effects　of　the　fuel-assembly　bowing,　our　home-developed　Bamboo-C　code　system　based　on　the　two-step　scheme　has　been　improved.　In　the　lattice　code　LOCUST,　the　few-group　constants　were　generated　with　different　sizes　of　water　gaps　around　the　fuel　assembly.　In　the　core　code　SPARK,　the　neutron-diffusion　solver　has　been　improved　to　consider　the　geometry-grid　variations　due　to　the　whole-core　fuel-assembly　bowing.　For　the　method　verifications,　two　test　cases　of　HPR1000　with　different　kinds　of　fuel-assembly　bowings　have　been　designed　and　simulated,　with　corresponding　reference　results　provided　by　the　Monte-Carlo　code　NECP-MCX.　The　verification　results　show　that　well-agreed　eigenvalue　and　assembly-averaged　power　distributions　can　be　provided　by　the　improved　Bamboo-C　code.　©　2022