Metal-doped　(Cu,　Zn,　Mn)　g-C3N4　was　synthesized　by　a　simple　high-temperature　process,　followed　by　the　insertion　of　one-dimensional　nanofibrillar　cellulose　(CNF)　into　the　two-dimensional　g-C3N4.　Photocatalytic　composite　membranes　were　then　prepared　using　a　vacuum-assisted　filtration　method.　A　series　of　characterization　techniques,　including　XRD,　SEM,　FT-IR,　and　UV–vis　DRS,　were　employed　to　systematically　analyze　the　microstructure,　chemical　composition,　and　physicochemical　properties　of　the　designed　g-C3N4/CNF　composite　membranes.　The　results　indicated　that　the　visible　photocatalytic　activity　of　the　metal-doped　photocatalysts　was　enhanced,　which　is　beneficial　for　pollutant　degradation　by　reducing　the　bandgap　and　extending　the　absorption　of　visible　light.　Notably,　the　composite　membrane　prepared　with　Mn-doped　g-C3N4　demonstrated　the　highest　photocatalytic　performance　in　degrading　rhodamine　B　dye,　achieving　a　42.6%　degradation　rate　within　7 h.　Additionally,　the　water　flux　and　retention　rate　of　the　composite　membranes　were　improved　after　metal　doping,　with　Zn-doped　g-C3N4　showing　approximately　six　times　the　water　flux　of　undoped　g-C3N4,　reaching　a　rate　of　293.64　L·m−2·h−1·bar−1.　Graphic　abstract:　(Figure　presented.)　©　The　Author(s),　under　exclusive　licence　to　Springer　Nature　Switzerland　AG　2024.