The classical rough set theory is not directly to deal with incomplete information system, therefore, the rough set model of tolerance relation, non-symmetric similarity relation, limited tolerance relation, limited non-symmetric similarity relation and limited non-symmetric similarity relation model based on object difference degree have been proposed. Through the analysis of the advantage and disadvantage of these models, the new definition of the degree difference between objects, it puts forward a new extension of rough set which is limited tolerance relation model based on degree of difference between objects, the results show that the proposed model more accurate, more practical.