The　threat　of　HEMP　(high　altitude　nuclear　electromagnetic　pulse)　in　power　system　has　emerged　significant　to　be　assessed.　The　thyristor　converter　valve　is　the　core　equipment　in　UHVDC　(ultra-high　voltage　direct　current)　transmission　system,　and　to　evaluate　its　reliability　in　HEMP　environment　is　quite　important.　The　failure　rate　of　thyristor　is　the　critical　parameter　for　the　overall　reliability　evaluation,　while　its　value　varies　depending　on　the　electrical　and　thermal　stress　conditions.　This　paper　proposed　a　modeling　method　for　the　calculation　of　failure　rate　of　thyristor.　First,　the　wideband　model　of　key　components　as　well　as　stray　capacitances　in　the　converter　valve　is　built　based　on　impedance　measurement.　Second,　thermal　impedance　model　of　thyristor　is　built,　and　the　model　parameters　are　calculated　according　to　the　transient　thermal　impedance　curve　of　the　device.　Based　on　the　models　above,　the　electrical　and　thermal　stresses　of　thyristor　are　obtained　and　analyzed.　Finally,　the　reliability　model　of　thyristor　is　established,　and　the　parameter　failure　rate　is　calculated　which　could　be　used　in　the　further　study　of　converter　valve　reliability.　The　quantitative　analysis　is　presented　in　the　paper.