In the traditional community classification system, Association is a basic unit of plant community classification system. the necessary of association classified is put forward at first, the concept and classification method of association in traditional plant community classification system are summarized,that is, dominant species or characteristic species(group) in different layers of community are main evidence for dividing association. But there are some uncertained factors for dividing association in traditional classification system, such as, characteristic species(group) for association are affirmed artificially.On the other hand, research state of community quantitative classification today is narrated, TWINSPAN, PCA are mian methods for plant classification,and some problem when them are used. The relations between the results of quantitative classification and traditional community classification unit aren't unanimous, it is difficult to judge whether associations have been divided. At last, prospects of association classification are brought: quantitative methods are basic, characteristic species(group) and its quantitative character are key.