One　of　the　main　reasons　for　the　failure　of　transformer　bushings　is　moisture　penetration.　Here,　an　online　monitoring　method　of　moisture　in　transformer　bushings　based　on　an　improved　humidity　indicator　card,　which　belongs　to　the　group　of　passive　physical　and　chemical　monitoring　methods,　is　proposed.　First,　the　operating　principles　and　monitoring　system　are　explained.　Next,　by　means　of　a　moisture　simulation　device,　the　moisture　migration　characteristics　between　the　gas　and　oil,　the　influence　of　the　interface　radius　on　the　response　characteristics　of　the　monitoring　device,　and　the　characteristics　of　the　humidity　indicator　card　are　studied　experimentally.　Then,　the　moisture　migration　process　in　micro-scale　is　simulated　by　molecular　dynamics　simulation.　Finally,　by　changing　the　mass　fraction　of　the　cobalt　chloride　in　the　humidity　card,　the　warning　values　of　relative　humidity　at　different　temperatures　are　obtained.　The　simulation　results　show　that　water　molecules　can　migrate　from　the　oil–paper　system　to　the　air.　Moreover,　once　the　moisture　enters　the　transformer　bushing,　the　proposed　humidity　indicator　card　can　correctly　display　the　relative　humidity　at　different　temperatures.　In　conclusion,　the　proposed　method　provides　a　new　way　for　online　moisture　monitoring　of　transformer　bushings.　©　2021　The　Authors.　IET　Generation,　Transmission　&　Distribution　published　by　John　Wiley　&　Sons　Ltd　on　behalf　of　The　Institution　of　Engineering　and　Technology