It's common that general rules exist in a certain classification. The general rules of expense classification enable us to judge the category of a patient as soon as possible and to curb the expense. Theory of rough set helps us reach the best reduction of attributes. Based on the core attributes, classification rules are put forward by value reduction. The results show that 10 core attributes remain in 21 attributes of 1527 inpatients' information and 76 classification rules are founded. All of 76 rules guide classification of the patients. 44 of the 76 rules define the only category of a patient, the other 32 rules defines the potential catagories of a patient. Meanwhile, equal attributes of the same category are summerized to guide the cost control of patients. The results indicate that the theory of rough set is effective in attributes reduction and rule generalization of patient expense classification, and it has important significance on medical practice.