Solar　energy　harvesting　and　conversion　into　useful　chemical　energy　with　the　aid　of　semiconductor　photocatalysts　is　a　promising　technique　to　solve　both　energy　and　environmental　issues.　This　work　reports　a　successful　synthesis　of　CdSe　quantum　dots　(QDs)　modified　phosphorus　doped　g-C3N4　(P-CN)　for　advanced　photocatalytic　applications.　Phosphorus　doping　and　structural　coupling　with　CdSe　QDs　are　shown　to　significantly　extend　visible-light　response　of　g-C3N4　up　to　700　nm.　The　optimized　sample　4CdSe/P-CN　demonstrates　enhanced　visible-light　driven　overall　water　splitting　activities　for　H-2　and　O-2　evolution　i.e.　113　and　55.5　mu　mol.h(-1).g(-1),　respectively,　as　well　as　very　high　photocatalytic　CO2　to　CH4　conversion　efficiency　(47　mu　mol.h(-1).　g(-1)).　It　also　exhibit　higher　activity　(78　%)　for　2,4-dichlorophenol　degradation　as　compared　to　pristine　CN-sample.　Combined　photoluminescence,　transient/single　wavelength　photocurrent,　photoelectrochemical,　and　coumarin　fluorescence　spectroscopy　demonstrate　that　4CdSe/P-CN　nanocomposite　exhibit　enhanced　charge　separation　efficiency　which　is　responsible　for　improved　visible　light　catalytic　activities.　Our　work　thus　provide　a　new　strategy　to　design　low-cost　and　sustainable　photocatalysis　with　wide　visible-light　activity　for　practical　overall　water　splitting　and　CO2　reduction　applications.