Aiming at the problem of equivalent conversion from conjunctive normal form to disjunctive normal form, an effective algorithm was proposed to construct reduction tree based on discernibility matrix. The discernibility set was acquired by improving discernibility matrix, and then reduction tree was designed to describe concrete process of getting reductions, finally each path from root to leaf was set to a reduction. This method decreases overhead in forming and storing discernibility matrix, and simplifies the process of acquiring all reductions of decision system.