The　grain　boundary　diffusion　(GBD)　method　could　effectively　improve　the　coercivity　of　Nd-Fe-B　sintered　magnets,　but　the　uneven　distribution　of　heave　rare　earth　(HRE)　elements　would　lead　to　the　decrease　of　squareness　factor　of　the　demagnetization　curve　of　the　magnets,　which　limited　the　application　of　this　method.　In　this　paper,　three　typical-structure　regions　were　summarized　based　on　microstructure　characterization,　and　the　magnetic　properties　and　magnetization　reversal　mechanism　were　analyzed.　The　differences　in　nucleation　positions　and　magnetization　reversal　processes　of　the　three　regions　were　discussed,　and　it　found　that　the　remarkable　differences　among　them　resulted　in　the　low　squareness　factor　of　GBD　Nd-Fe-B　sintered　magnets.　In　addition,　the　efficiency　of　Tb　elements　could　be　further　improved　to　enhance　the　coercivity　of　Nd-Fe-B　sintered　magnets　by　forming　homogeneous　and　complete　core-shell　structure　grains,　which　could　strengthen　the　nucleation　field　and　even　transfer　the　nucleation　position　from　the　grain　surface　defect　layer　to　the　grain　interior　region.　Our　results　could　provide　guidance　and　reference　for　further　improving　the　permanent　magnetic　properties　of　Nd-Fe-B　sintered　magnets　by　optimized　GBD　method.