The　effect　of　bipolar　pulse　mode　current　ratio　(anodic　to　cathodic)　on　plasma　emission　behavior　was　investigated　in　Plasma　Electrolytic　Oxidation　(PEO)　on　AZ91D　Mg-Alloy.　Two　cases　of　current　ratio　as　1.20　and　0.88　were　applied　to　the　sample.　Plasma　emission　behavior　was　studied　using　plasma　images　and　plasma　emission　measured　by　photodetector　and　Intensified　Charged-Couple　Device　(ICCD)　camera.　The　current　ratio　of　greater　than　1　shows　the　continuous　increase　and　then　stabilization　in　emission　intensity　with　a　gradual　increase　in　voltage　throughout　the　PEO　process.　In　contrast,　the　current　ratio　of　less　than　1,　a　sudden　drop　in　plasma　emission　intensity　with　voltage　was　found　after　786　s.　Therefore,　PEO　process　can　be　divided　into　two　regimes,　arc　regime　and　soft　regime,　before　and　after　voltage　drop,　respectively.　Results　of　measured　spectra　show　that　a　soft　regime　does　not　have　atomic　or　ionic　excitation　during　PEO　process.　It　is　demonstrated　that　the　growth　of　porous　layer　during　PEO　can　be　controlled,　which　is　benefit　for　the　protective　oxide　coating　of　sample.