With　the　widespread　application　of　ubiquitous　computing,　the　context-aware　computing　is　introduced　into　virtual　environment　for　providing　proactive　service　model　in　this　paper.　The　key　technology　of　context-aware　computing　is　to　predefine　the　context　rules　set　for　inference.　The　current　generation　methods　of　context　rules　however　are　usually　dependent　on　manual　definition,　which　could　cause　lots　of　problems.　Therefore　the　modified　ID3　algorithm　is　proposed　to　automatically　generate　context　rules　based　on　virtual　environment　in　this　paper.　It　is　to　build　a　context　decision　tree　and　then　convert　this　tree　into　rules　set.　The　experimental　result　shows　that　this　algorithm　has　a　good　performance　in　effectiveness　of　generated　rules　and　computational　efficiency.