The data in information system is dynamically changed.How to acquisite useful information according to dynamical varied information system is a key problem in data processing. To deal with the problem, the approaches for dynamical approximations acquisition while adding or deleting an attribute are respectively discussed in information system. By dividing original equivalent classes in information systems, an approach which avoids re-division of the universe is proposed. The efficiency of dynamical updating approximation is improved. By analyzing the relationship between equivalent classes and original approximations, the corresponding theorems between updated approximations and original approximations are given. Then, the approaches for dynamical acquisition of approximations while adding or deleting an attribute are respectively proposed in classical rough set model. Experimental results verify the validity of the approaches and prove that the efficiencies of the proposed approaches are better than those of the original approaches.