As　the　population　of　transformers　in　service　increases　and　becomes　older,　more　attentions　are　focused　on　their　availability　and　reliability.　Through　decades＇　progress,　it　is　undeniable　that　considerable　advances　have　been　achieved　on　the　techniques　of　fault　diagnosis　and　condition　assessment　for　oil-immersed　transformer,　for　its　great　importance　in　the　whole　transmission　and　distribution　(T&D)　system.　However,　still　insufficient　research　work　focused　on　how　to　evaluate　transformer＇s　aging　condition.　This　paper　represents　Aging　Index　(AI)　which　is　a　practical　tool　that　combines　the　results　of　routine　inspections,　and　site　and　laboratory　testing　to　estimate　the　aging　condition.　This　Aging　Index　calculation　considers　not　only　typical　test　results　such　as　dissolved　gas　analysis　(DGA),　oil　quality,　furan,　and　dissipation　factor,　but　also　other　parameters　such　as　top　oil　temperature　and　hot　spot　temperature　estimating.　And　further,　Frequency　Domain　Spectroscopy　(FDS)　is　also　introduced　as　a　novel　non-destructive　testing　technique　to　estimate　remained　life　expectancy.　Moreover,　a　software　system　based　on　transformer＇s　electrical　and　thermal　parameters　is　developed　correspondingly,　by　using　a　multi-parameters　analytic　approach.　This　system　is　expected　to　help　to　decision-making　and　replacement　planning　and　to　have　good　application　prospects.