Monolayer　CrI3　with　robust　ferromagnetism　has　been　extensively　studied.　Its　Janus　structures　could　further　enhance　its　applications　in　spintronic　devices.　Here　we　investigate　the　intrinsic　defects　and　their　induced　magnetic　properties　in　Janus　structures.　We　constructed　the　point　defects　in　Janus　CrX1.5Y1.5　(X,　Y　=　Cl,　Br,　I;　X　≠　Y)　monolayer,　and　studied　the　structural　stability　and　migration　barriers　by　first-principles　calculations.　The　single　vacancy　with　one　halogen　atom　missing,　is　the　most　stable　defect　in　Cr-rich　condition,　while　the　Cr　vacancy　is　preferred　to　form　in　halogen-rich　condition.　Our　results　revealed　that　the　surface　defects　not　only　enhance　the　intrinsic　ferromagnetism　of　the　Janus　structures,　but　also　increase　the　Curie　temperatures　effectively.　The　intriguing　properties　induced　by　the　surface　defects　make　the　Janus　chromium　trihalides　monolayer　promising　for　multifunctional　nanodevices.　©　2021