Figuring　out　the　effects　of　LPSO　morphology　on　the　deformation　mode　and　strengthening　mechanism　of　Mg　-RE　alloys　is　helpful　to　improve　their　formability　and　mechanical　properties.　In　this　work,　the　Mg-8Gd-1Er-1Zn-0.6Zr　(GEZ811)　alloys　with　monolithic　block　LPSO　(B-alloy)　and　lamellar　LPSO　(L-alloy)　phase　were　respectively　prepared　by　different　heat　treatment　route.　The　effects　of　the　monolithic　LPSO　morphology　on　deformation　mode　and　strengthening　mechanisms　during　high　strain　rate　multidirectional　forging　(HSMDF)　were　analyzed.　Results　showed　at　the　initial　stage　of　HSMDF,　twinning　was　the　dominant　deformation　mode　for　B-alloy　while　non　-basal　slip　was　for　L-alloy.　With　increasing　forging　pass,　kink　band　of　grains　became　primary　for　B-alloy,　whereas　kinking　and　breaking　of　the　lamellar　LPSO　phase　was　dominant　for　L-alloy.　The　optimal　UTS,　YS　and　EL　of　the　HSMDFed　B-alloy　and　L-alloy　at　12　P　were　381.6　MPa,　324.1　MPa,　and　8.8　%,　and　358.4　MPa,　295.0　MPa,　and　5.5　%,　respectively.　The　difference　in　strength　between　B-alloy　and　L-alloy　resulted　primarily　from　the　contribution　of　grain　boundary　strengthening　and　stacking　fault　strengthening,　while　the　better　plasticity　of　B-alloy　was　attributed　to　its　lower　deformation　activation　energy　and　the　formed　kink　band　of　grains.　Retaining　the　block　LPSO　phase　as　much　as　possible　in　the　alloy　before　deformation　is　beneficial　to　enhancing　the　mechanical　performance　of　Mg　-RE　alloys.