To　improve　the　dynamic　recovery　characteristic　of　ultra　high　voltage　dc　(UHVDC)　transmission　system,　this　paper　focuses　on　effects　of　nonlinear　optimal　control　and　PI　control　in　transient　recovery　process.　Control　systems　in　UHVDC　based　on　state　feedback　exact　linearization　method　and　PI　controllers　are　respectively　established.　The　rationality　and　validity　of　UHVDC　transmission　system　on　resisting　disturbances　are　verified　by　simulation　results　using　PSCAD/EMTDC　software.　Compared　with　PI　controllers,　nonlinear　optimal　control　can　keep　constant　current　and　voltage　more　accurately,　shorten　system　recovery　time　about　50%,　realize　the　decoupling　control,　are　more　useful　for　system　stability　of　UHVDC　and　ac　networks.　©　2019　IEEE.