The　absorption　and　the　emission　spectra　of　wurtzite　GaN　with　some　vacancy　defects　or　impurities　(V-Ga,　V-Ga-O-N,　V-Ga-H,　V-N,　V-N-H,　C-Ga,　C-N,　C-i,C-　Si-Ga,　O-N)　were　calculated　by　using　TDDFT　method　with　cluster　models.　Ga26N26H50　cluster　is　chosen　as　a　basic　cluster　according　to　the　predicted　energy　gap.　The　TD-B3LYP　calculation　results　show　that　yellow　emission　of　about　2.2　eV　of　GaN　material　attributes　to　the　presence　of　V-Ga-O-N　complex.　The　other　vacancy　defects　or　impurities　bring　out　emission　of　about　2.0-2.1　eV　or　have　no　effect　on　yellow　emission　of　GaN.