The　work　function　of　a　silicon　substrate　will　decrease　rapidly　and　a　negative　electron　affinity　Si　photocathode　may　be　produced　when　a　clean　Si(100)　substrate　is　activated　by　applying　cesium　and　then　oxygen,　only　once.　But　the　work　function　of　thus　produced　Si(100)　photocathode　would　be　increased　and　its　integral　photoemission　sensitivity　would　be　decreased　if　the　cathode　is　subsequently　treated　with　Cs　and/or　O2.　In　this　letter,　a　new　activation　technology　is　adopted,　and　a　reflection　mode　negative　electron　affinity　(Si-Na2KSb-Cs)-O-Cs　photocathode　with　maximum　photoemission　sensitivity　S　=　2000-mu-A/1m　is　fabricated.　The　surface　layer　model　and　the　energy-band　structure　of　the　photocathode　are　also　discussed.