We　identify　a　valley-polarized　Chern　insulator　in　a　van　der　Waals　heterostructure,　monolayer　Pt2HgSe3/monolayer　CrI3,　for　potential　applications　with　interplay　between　electric,　magnetic,　optical,　and　mechanical　effects.　The　interlayer　proximity　magnetic　coupling　nearly　closes　the　band　gap　of　monolayer　Pt2HgSe3,　and　the　strong　intralayer　spin-orbit　coupling　further　lifts　the　valley　degeneracy　by　over　100　meV,　leading　to　positive　and　negative　band　gaps　at　opposite　valleys.　In　the　valley　with　negative　gap,　the　interfacial　Rashba　spin-orbit　coupling　opens　a　topological　band　gap　of　17.8　meV,　which　is　enlarged　to　30.8　meV　by　adding　a　hexagonal　boron　nitride　(h-BN)　layer.　We　find　large　orbital　magnetization　in　the　Pt2HgSe3　layer　that　is　much　larger　than　spin,　which　can　induce　a　measurable　optical　Kerr　effect.　The　valley　polarization　and　Chern　number　are　coupled　to　the　magnetic　order　of　the　nearest-neighbor　CrI3　layer,　which　is　switchable　by　electric,　magnetic,　and　mechanical　means　in　experiments.　The　presence　of　h-BN　protects　the　topological　phase,　allowing　the　construction　of　superlattices　with　valley,　spin,　and　layer　degrees　of　freedom.　©　2021　American　Physical　Society.