Activity support is one of eight elements which Hamid Shirvani has generalized. It is also the element which is the most direct with the relationship of people’s activities and the most difficult with operation and description in the eight elements. While Extenics is that we use the formal model to study the possibilities of expansion and the rules and methods of innovation. Extenics can make use of basic-element model to describe everything. So we can do formal analysis on the objects of study, such as activity support element. Based on activity support element, the article is using the concepts of matter-element, affair-element and relation-element in Extenics to analyze activity support element, and using the principle of divergence analysis to do expansion analysis, which can make the activity support element systematically integrate the other urban design elements among the correlative net. There is the methodological significance with scientifically processing the relationship of behavior and environment in the article.