The problem of grey multi-attribute group decision making under risk, in which the attributes are interval grey number and the attribute weights are unknown is studied. Based on the ideal matrix, relative subordinate degree method is presented. Combining thoughts and method of grey system theory with classical risk decision-making method, the attribute of each alternative is obtained by grey relation degree. Then the relative subordinate degree in which an alternative corresponds to the ideal matrix is obtained by means of the synthetically weighted distance among the alternative and positive and the negative matrix. An example illustrates the validity and efficiency of this method.