The upper and lower approximation sets of rough sets derived from approximate space are the core foundation of the rough set theory and its applied research. However， this description only gives a range of rough sets. To depict the target concept and promote rough set application more accurately， this paper， according to the computation problem of the optimal approximate set of rough sets， proposed a calculation method based on matrixes. Then， some concepts such as Boolean matrix， relation matrix， set matrix and similarity degree matrix were introduced. By discussing the similarity degree between the target concept and its approximate set， some relevant rules were obtained. A new judgment and algorithm of optimal approximate set based on matrix for rough set was eventually established. Finally， a practical example was given to further illustrate results above.