Attribute reduction is a key problem in theoretical research of rough set. Aiming at the NP-hard problem of acquiring all decision reduction in decision table, the paper simplifies the problem to decision reduction computation in incremental object，The arithmetic of attribute reduction based on minimal disjunctive normal form is respectively offered on the basis of analyzing the characteristic and correlation of discernibility set in discernibility matrix, in which the decision discernibility matrix is optimized by ① omitting repeated contrast between the objects; ② transforming the row of the decision discernibility matrix to the equivalent set; ③ treating the equivalent set of the positive field as the other row of the decision discernibility matrix. The method offers a tool to uniformly acquire all decision reduction of consistent and inconsistent decision table. The example analysis shows this algorithm's feasibility and validity, which offers an effective way for attribute reduction.