Abstract: In classical rough set theory，a target concept can usually be depicted via static granulation，which cannot well characterize dynamic cognitive process in problem solving.A target concept and a target decision were approximated using positive approximation and converse approximation，respectively，and they have been effectively applied in clustering algorithm and rule extracting.Based on dynamic granulation principle，the concept of bidirectional approximation is proposed，some important properties are obtained.They have been applied for extracting decision rules from decision table in rough set theory.

摘要： 经典的粗糙集理论刻画目标概念运用静态的粒度分析，不便于刻画人们问题求解的动态认知过程。已有文献分别用正向近似和逆向近似对目标概念和目标决策进行刻画，并成功地应用于分层聚类算法和规则提取方面。基于动态粒度原理，提出双向近似的概念，获得双向近似的一些重要性质，并将其应用于决策表中决策规则的获取。