Investigation　on　effects　of　hydrogen　addition　fraction　on　combustion　and　emission　characteristics　of　SI　gasoline　engine　was　performed　on　a　modified　SI　engine　equipped　with　an　electron-controlled　hydrogen　port-injection　system,　under　the　conditions　of　the　excess　air　ratio　of　1.1　and　engine　speed　1400　r/min.　The　hydrogen　volumetric　fraction　in　the　total　intake　air　was　gradually　increased　from　0　up　to　6%　by　adding　hydrogen　injection　pulse　width.　The　experiment　results　demonstrate　that　the　engine　brake　thermal　efficiency　(BTE)　is　improved　by　hydrogen　enrichment.　And　when　the　hydrogen　volumetric　fraction　reaches　6%,　the　BTE　increases　by　14.53%　on　average,　compared　to　fueled　with　pure　gasoline.　The　ignition　delay　and　fast　burning　duration　shorten,　the　peak　cylinder　pressure　rises　and　its　relevant　crank　angle　advances,　while　the　cyclic　variation　in　peak　cylinder　pressure　decreases　with　increasing　of　hydrogen　enrichment　level.　At　low　hydrogen　addition　fraction,　CO　emission　decreases　with　the　hydrogen　enrichment　level　increasing,　but　as　hydrogen　volumetric　fraction　reaches　6%,　CO　emission　increases　again.　HC　emission　reduces　and　NOx　emission　increases　with　the　hydrogen　addition.