Carbon　dots　(CDs),　as　a　new　zero-dimensional　carbon-based　nanomaterial　with　desirable　optical　properties,　exhibit　great　potential　for　many　application　fields.　However,　the　preparation　technique　of　multiple　emission　CDs　with　high　yield　is　difficult　and　complex.　Therefore,　exploring　the　large-scale　and　straightforward　synthesis　of　multicolor　CDs　from　a　simple　carbon　source　is　necessary.　In　this　work,　the　solvent-free　method　prepares　a　series　of　multicolor　emission　CDs　from　dihydroxybenzoic　acid　(DHBA).　The　maximum　emission　wavelengths　are　408,　445,　553,　580,　and　610　nm,　respectively,　covering　the　visible　light　region.　The　2,4-　and　2,6-CDs　possess　the　longer　emission　wavelength　caused　by　the　2,4-,　and　2,6-DHBA　easily　undergo　decarboxylation　to　form　the　larger　sp(2)　domain　graphitized　structure.　These　CDs　incorporated　with　g-C3N4　can　significantly　improve　the　photocatalytic　water-splitting　hydrogen　production　rate　by　extending　the　visible　light　absorption　and　enhancing　the　charge　separation　efficiency.　The　long-wavelength　emission　CDs　can　further　enhance　photocatalytic　activity　primarily　by　improving　visible　light　absorption　efficiency.