To　improve　the　solar　energy　conversion　of　GaN　for　photocatalytic,　the　band　gap　of　GaN　should　be　tailored　to　match　with　visible　light　absorption,　we　calculated　that　Cr/C　codoping　effectively　narrows　the　band　gap　of　GaN　by　using　first-principles　calculations.　Cr/C　codoping　can　split　the　band　gap　with　the　formation　of　an　intermediate　band.　Positive　defect　pair　binding　energy　indicates　that　the　defect　pairs　are　stable　with　respect　to　the　isolate　impurity　in　the　sample.　The　band　gap　narrowing　can　be　optimized　to　shift　light　absorption　into　the　peak　of　the　visible　spectral　region,　which　enable　maximal　utilization　of　sunlight　and　thus　offer　immense　potential　for　applications　in　solar　energy　conversion.