In　accordance　with　slow　running　speed　in　present　soft　CNC　system,　which　is　difficult　to　be　adapted　to　machining　with　mass　of　data　at　high　speed,　an　open　architecture　of　soft　CNC　system　with　high　performance　is　presented,　which　is　based　on　PC+I/O　hardware　interface　and　satisfies　the　requirement　of　real　time　performance　in　data　transmission　by　use　of　the　fifos　of　RTAI,　of　which　modules　are　redundantly　designed　to　make　its　functions　reconstructable,　extensible　and　open　by　use　of　the　routing　technology　of　soft　switch.　In　this　system,　functions　of　fine　interpolation　and　movement　control,　a　small　portion　of　CNC　system＇s　functions,　are　performed　by　the　designed　simple　I/O　hardware　interface.　It　is　expected　to　greatly　improve　the　real-time　performance　and　satisfy　the　requirement　of　high　running　speed　in　CNC　system.