High　voltage　switchgear　is　prone　to　moisture　and　condensation　in　environments　with　high　humidity　and　large　temperature　difference,　which　seriously　affects　its　safe　and　reliable　operation.　This　paper　uses　three-dimensional　numerical　simulation,　experimental　verification,　and　mechanism　analysis　to　examine　the　moisture　condensation　problem　inside　high　voltage　switchgear.　First,　a　three-dimensional　electromagnetic-fluid-thermal-humidity　coupling　field　simulation　model　of　full-size　40.5　kV　switchgear　is　established,　and　the　temperature　distribution　and　humidity　diffusion　process　are　calculated.　Then,　a　refined　measurement　is　performed　on　the　temporal　and　spatial　distribution　of　temperature　and　humidity　inside　the　switchgear　to　validate　the　simulation　results.　Finally,　the　condensation　development　sequence　of　each　room　and　insulating　component　is　revealed　for　the　first　time　by　combining　the　natural　penetration　test　and　simulation　results.　The　results　show　that　the　relative　errors　of　temperature　and　humidity　data　between　the　simulation　and　experimental　results　are　within　+/-　6%,　which　verifies　the　three-dimensional　numerical　simulation.　Condensation　is　easy　to　occur　on　insulating　components,　including　the　current　transformer　and　contact　box　in　the　cable　room,　where　the　humid　air　first　accumulates.　Furthermore,　the　condensation　formation　time　decreases　with　the　increase　of　relative　humidity　or　temperature　difference.　The　research　results　can　provide　reference　for　the　anti-condensation　measures　for　high　voltage　switchgear.