Precisely　modulating　the　interfacial　contact　of　heterojunction　photocatalysts　and　regulating　the　spatial　transfer　of　photoexcited　charge　carriers　are　the　core　factors　to　promote　the　photocatalytic　efficiency　for　energy　and　environmental　applications.　Herein,　novel　2D/2D　relative　p-n　heterojunction　CdS/g-C3N4　with　Z-scheme　(or　S-scheme)　was　fabricated　by　in-situ　growing　ultrasmall　2D　CdS　nanoflakes　on　the　surface　of　2D　g-C3N4　nanosheets.　The　optimized　2D/2D　heterojunction　structure　with　proper　band　configuration　and　suitable　heterojunction　interface　can　improve　photoexcited　charge　carriers　separation　spatially　and　transfer　to　the　ideal　reaction　sites.　Therefore,　2D/2D　CdS/g-C3N4　composites　show　high　photocatalytic　reduction　(H-2　evolution)　and　oxidation　(selective　oxidation　of　5-hydroxymethylfurfural　into　2,5-diformylfuran)　activities,　which　are　about　26.8　times　and　3.51　times　higher　than　individual　g-C3N4　and　are　about　4.4　times　and　2.39　times　higher　than　individual　CdS,　respectively.　The　mechanisms　including　direction　and　inner　impetus　of　the　photoexcited　charge　separation　and　transfer　in　the　composite　photocatalyst　are　proposed　and　verified　by　experimental　and　computational　analyses.　This　investigation　highlights　the　promising　scope　to　intelligently　regulate　the　spatial　separation　and　transfer　of　photoexcited　charge　carriers　and　rationally　construct　new　relative　p-n　junction　or　Z-scheme　photocatalysts　for　energy　and　environmental　applications.