Fault　diagnosis　has　a　critical　role　in　maintaining　the　reliability　and　stability　of　distribution　networks.　Compared　with　common　diagnosis　methods,　such　as　expert　systems　that　depend　on　the　professional　experience　and　information　of　fault,　the　model-based　diagnosis　(MBD)　has　many　distinct　advantages.　However,　the　efficiency　and　accuracy　of　the　calculation　of　the　minimum　hitting　set　(MHS)　limit　the　performance　of　MBD.　To　overcome　these　limitations,　an　MHS　algorithm　with　a　prejudging　mechanism　is　proposed　in　this　article.　This　mechanism　is　constructed　based　on　the　distribution　characteristics　of　MHS.　The　use　of　this　mechanism　prior　to　MHS　calculation　enables　the　removal　of　most　non-HS　particles　and　all　super-MHS　particles　and　enables　the　compression　of　the　MHS　calculation　solution　space　several　times.　Moreover,　after　compression,　only　two　types　of　particles　remain　in　the　left　solution　space,　which　are　non-HS　and　MHS.　As　a　result,　the　MHS　can　be　picked　out　directly　through　the　HS　fitness　function　and　the　HS　criterion　and　by　avoiding　the　participation　of　the　MHS　ensured　strategy　(MHSES).　Three　cases　studies　have　been　introduced　to　test　the　effect　of　the　prejudging　mechanism　on　the　performance　of　the　MHS　calculation.　In　addition,　a　discussion　on　the　MBD　of　the　distribution　networks　has　been　presented　to　validate　the　efficiency　and　accuracy　of　the　proposed　algorithm.　©　1963-2012　IEEE.