The　corona　discharge　of　some　grading　rings　is　serious　on　500　kV　compact　transmission　line　in　China,　due　to　the　improper　structure　design　and　installation.　In　order　to　suppress　the　corona　and　avoid　the　same　fault　in　the　future,　it　is　necessary　to　optimize　the　structure　of　the　grading　ring.　Consequently,　the　grading　rings　having　obvious　corona　discharge　were　found　out　by　observing　hardware　fittings　along　lines　with　UVI　(ultraviolet　imager),　then　the　electric　field　distribution　of　the　grading　rings　was　calculated　using　three-dimension　FEM(finite　element　method).　After　analyzing　the　electric　field　distribution　to　get　the　causes　of　corona　discharge,　the　structure　of　grading　ring　was　optimized　to　reduce　the　maximum　electric　field　intensity　on　its　surface　from　3868　V/mm　to　2475　V/mm.　According　to　the　optimization　proposals,　new　grading　rings　were　manufactured　and　hung　on　operating　lines.　The　observations　by　UVI　show　that　the　new　ones　have　no　corona　discharge　on　their　spheres　and　bodies　which　validate　the　optimizations.　This　study　can　provide　experiences　and　references　for　the　design　of　anti-corona　hardware　fittings　on　high　voltage　transmission　lines.